You will be given a definition of a task first, then some input of the task.
In this task, you're given passages that contain mentions of names of people, places, or things. Some of these mentions refer to the same person, place, or thing. Your job is to write questions that evaluate one's understanding of such references. Good questions are expected to link pronouns (she, her, him, his, their, etc.) or other mentions to people, places, or things to which they may refer. Do not ask questions that can be answered correctly without understanding the paragraph or having multiple answers. Avoid questions that do not link phrases referring to the same entity. For each of your questions, the answer should be one or more phrases in the paragraph, and it should be unambiguous.

Passage: Neanderthal remains dating to around 150,000 years ago, or the Middle Paleolithic, are some of the earliest traces of human activity in the lands of modern Bulgaria. The Karanovo culture arose circa 6,500 BC and was one of several Neolithic societies in the region that thrived on agriculture. The Copper Age Varna culture (fifth millennium BC) is credited with inventing gold metallurgy. The associated Varna Necropolis treasure contains the oldest golden jewellery in the world with an approximate age of over 6,000 years. The treasure has been valuable for understanding social hierarchy and stratification in the earliest European societies.The Thracians, one of the three primary ancestral groups of modern Bulgarians, appeared on the Balkan Peninsula some time before the 12th century BC. The Thracians excelled in metallurgy and gave the Greeks the Orphean and Dionysian cults, but remained tribal and stateless. The Persian Achaemenid Empire conquered most of present-day Bulgaria in the 6th century BC and retained control over the region until 479 BC. The invasion became a catalyst for Thracian unity, and the bulk of their tribes united under king Teres to form the Odrysian kingdom in the 470s BC. It was weakened and vassalized by Philip II of Macedon in 341 BC, attacked by Celts in the 3rd century, and finally became a province of the Roman Empire in AD 45.By the end of the 1st century AD, Roman governance was established over the entire Balkan Peninsula and Christianity began spreading in the region around the 4th century. The Gothic Bible—the first Germanic language book—was created by Gothic bishop Ulfilas in what is today northern Bulgaria around 381. The region came under Byzantine control after the fall of Rome in 476. The Byzantines were engaged in prolonged warfare against Persia and could not defend their Balkan territories from barbarian incursions. This enabled the Slavs to enter the Balkan Peninsula as marauders, primarily through an area between the Danube River and the Balkan Mountains known as Moesia. Gradually, the interior of the peninsula became a country of the South Slavs, who lived under a democracy. The Slavs assimilated the partially Hellenized, Romanized, and Gothicized Thracians in the rural areas.
Output:
What has been valuable for understanding social hierarchy and stratification in the earliest European societies?