Given the task definition and input, reply with output. This task is about reading the given passage and construct a question about the information present in the passage. Construct a question in such a way that (i) it is unambiguous, (ii) it is answerable from the passage, (iii) its answer is unique (iv) its answer is a continuous text span from the paragraph. Avoid creating questions that (i) can be answered correctly without actually understanding the paragraph and (ii) uses same words or phrases given in the passage.

The main international airport serving Kathmandu and thus Nepal is the Tribhuvan International Airport, located about six kilometers (6 km (3.7 mi)) from the city centre. Operated by the Civil Aviation Authority of Nepal it has two terminals, one domestic and one international. At present, about 22 international airlines connect Nepal to other destinations in Europe, Asia and the Middle East, to cities such as Istanbul, Delhi, Kolkata, Singapore, Bangkok, Kuala Lumpur, Dhaka, Islamabad, Paro, Lhasa, Chengdu, and Guangzhou. A recent extension to the international terminal has made the distance to the airplanes shorter and in October 2009 it became possible to fly directly to Kathmandu from Amsterdam with Arkefly. Since 2013, Turkish Airlines connects Istanbul to Kathmandu. Regionally, several Nepali airlines operate from the city, including Agni Air, Buddha Air, Cosmic Air, Nepal Airlines and Yeti Airlines, to other major towns across Nepal.

What is Nepal's primary airport for international travel?