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.

Let me give you an example: Passage: Nearing London, Oliver encounters Jack Dawkins, a pickpocket more commonly known by the nickname the "Artful Dodger", and his sidekick, a boy of a humorous nature named Charley Bates, but Oliver's innocent and trusting nature fails to see any dishonesty in their actions. The Dodger provides Oliver with a free meal and tells him of a gentleman in London who will "give him lodgings for nothing, and never ask for change". Grateful for the unexpected assistance, Oliver follows the Dodger to the "old gentleman's" residence. In this way Oliver unwittingly falls in with an infamous Jewish criminal known as Fagin, the gentleman of whom the Artful Dodger spoke. Ensnared, Oliver lives with Fagin and his gang of juvenile pickpockets in their lair at Saffron Hill for some time, unaware of their criminal occupations. He believes they make wallets and handkerchiefs.
The answer to this example can be: Who believes Fagin's gang make wallets and handkerchiefs?.
Here is why: This question is based on the following sentence in the passage "He believes they make wallets and handkerchiefs". It evaluates the understanding that the pronoun "he" refers to name "Oliver". You can ask questions like this one about most pronouns in a paragraph.

OK. solve this:
Passage: Telephone services are widely available, and a central digital trunk line connects most regions. Vivacom (BTC) serves more than 90% of fixed lines and is one of the three operators providing mobile services, along with A1 and Telenor. Internet penetration stood at 61.9% of the population aged 16–74, in 2017.Bulgaria's strategic geographic location and well-developed energy sector make it a key European energy centre despite its lack of significant fossil fuel deposits. Thermal power plants generate 48.9% of electricity, followed by nuclear power from the Kozloduy reactors (34.8%) and renewable sources (16.3%). Equipment for a second nuclear power station at Belene has been acquired, but the fate of the project remains uncertain. Installed capacity amounts to 12,668 MW, allowing Bulgaria to exceed domestic demand and export energy.The national road network has a total length of 19,512 kilometres (12,124 mi), of which 19,235 kilometres (11,952 mi) are paved. Railroads are a major mode of freight transportation, although highways carry a progressively larger share of freight. Bulgaria has 6,238 kilometres (3,876 mi) of railway track and currently a total of 81 kilometres (50 miles) of high-speed lines are in operation. Rail links are available with Romania, Turkey, Greece, and Serbia, and express trains serve direct routes to Kiev, Minsk, Moscow and Saint Petersburg. Sofia and Plovdiv are the country's air travel hubs, while Varna and Burgas are the principal maritime trade ports.
Answer:
What is the name of the country whose air travel hubs are Sofia and Plovdiv?