Question: Information:  - Field artillery is a category of mobile artillery used to support armies in the field. These weapons are specialized for mobility, tactical proficiency, long range, short range and extremely long range target engagement.  - Infantry is the general branch of an army that engages in military combat on foot. As the troops who engage with the enemy in close-ranged combat, infantry units bear the largest brunt of warfare and typically suffer the greatest number of casualties during a military campaign. Historically, as the oldest branch of the combat arms, the infantry are the tip of the spear of a modern army, and continually undergo training that is typically more physically demanding and psychologically stressful than that of any other branch of the combat arms. Common representations of infantry fighting forces include the U.S. Army Infantry Branch, U.S. Marine Corps Infantry, Royal Canadian Infantry Corps, Infantry of the British Army, and the Royal Australian Corps of Infantry.  - Artillery is a class of large military weapons built to fire munitions far beyond the range and power of infantry's small arms. Early artillery development focused on the ability to breach fortifications, and led to heavy, fairly immobile siege engines. As technology improved, lighter, more mobile field artillery developed for battlefield use. This development continues today; modern self-propelled artillery vehicles are highly mobile weapons of great versatility providing the largest share of an army's total firepower.  - Fortifications are military constructions or buildings designed for the defense of territories in warfare, and also used to solidify rule in a region during peace time. Humans have constructed defensive works for many thousands of years, in a variety of increasingly complex designs. The term is derived from the Latin "fortis" ("strong") and "facere" ("to make").  - A siege is a military blockade of a city or fortress with the intent of conquering by attrition or assault. This derives from "sedere", Latin for "to sit". Siege warfare is a form of constant, low-intensity conflict characterized by one party holding a strong, static defensive position. Consequently, an opportunity for negotiation between combatants is not uncommon, as proximity and fluctuating advantage can encourage diplomacy.  - A tractor unit (prime mover or traction unit) is a characteristically heavy-duty towing engine that provides motive power for hauling a towed or trailered load. These fall into two categories: very heavy-duty typically off-road capable, often 6×6, military and commercial tractor units, including ballast tractors, and heavy and medium duty military and commercial rear-wheel drive "semi tractors" used for hauling semi-trailers.  - A ballast tractor is a specially weighted tractor unit of a heavy hauler combination. It is designed to utilize a drawbar to pull or push heavy or exceptionally large trailored loads. When feasible, lowboy-style semi-trailers are used to minimize a load's center of gravity. Typical drivetrains are 6×4 and 6×6.  - In international arms control, small arms include revolvers and pistols, rifles and carbines, assault rifles, submachine guns and light machine guns. Together with light weapons (heavy machine guns; hand-held grenade launchers; portable anti-aircraft and anti-tank guns; recoilless rifles; portable launchers of anti-aircraft and anti-tank missile systems; and mortars of calibres of less than 100 mm), they comprise the Small Arms and Light Weapons (SALW) protocol.  - A half-track is a civilian or military vehicle with regular wheels at the front for steering and continuous tracks at the back to propel the vehicle and carry most of the load. The purpose of this combination is to produce a vehicle with the cross-country capabilities of a tank and the handling of a wheeled vehicle.  - Self-propelled artillery (also called mobile artillery or locomotive artillery) is artillery equipped with its own propulsion system to move towards its target. Within the term are covered self-propelled guns (or howitzers) and rocket artillery. They are high mobility vehicles, usually based on continuous tracks carrying either a large howitzer, field gun, a mortar or some form of rocket or missile launcher. They are usually used for long-range indirect bombardment support on the battlefield.  - A semi-trailer (British English) or semitrailer (American English) is a trailer without a front axle. In the US, the term is also used to refer to the combination of a truck and a semi-trailer, a tractor-trailer.  - An artillery tractor , also referred to as a gun tractor , is a specialized heavy - duty form of tractor unit used to tow artillery pieces of varying weights and calibres . It may be wheeled , tracked , or half - tracked .  - A siege engine is a device that is designed to break or circumvent city walls and other fortifications in siege warfare. Some have been operated close to the fortifications, while others have been used to attack from a distance. From antiquity, siege engines were constructed largely of wood and tended to use mechanical advantage to fling stones and similar missiles. With the development of gunpowder and improved metallurgical techniques, siege engines became artillery. Collectively, siege engines or artillery together with the necessary troops and transport vehicles to conduct a siege are referred to as a siege-train.    Given the paragraphs above, decide what entity has the relation 'subclass of' with 'military vehicle'.
Answer:
artillery tractor