Read the following passage about a new kind of city, and mark the letter A, B, C, or D on your answer sheet to indicate the option that best fits each of the numbered blanks from 18 to 22.

Abu Dhabi, the capital of the United Arab Emirates, has a population of around 1.8 million. Its growing economy and population have led to traffic problems, (18) _______.
To solve this, a new city called Masdar is being built on the outskirts of Abu Dhabi. Covering 6 km², (19) _______. It gets all its electricity from solar power. A wall blocks desert wind, and narrow streets create shade and airflow, making the city about 10°C cooler than Abu Dhabi.
(20) _______. Instead, people rely on public transport, including underground rail and light-rail systems that connect Masdar to Abu Dhabi and the airport. A special transport system called Personal Rapid Transit (PRT) was also planned, featuring 3,000 driverless, solar-powered podcars. (21) _______. However, only 13 PRTs are now in use, as newer electric vehicle technology is more practical. Today, residents use clean-energy electric buses and cars alongside the PRT system.
Masdar was originally expected to cost $24 billion, but the 2008-2009 financial crisis reduced the budget. (22) _______. Still, construction continues, and Masdar is expected to be complete before 2030. When it is finished, there could be up to 50,000 residents and 40,000 daily commuters. With companies like Siemens opening offices there, planners are confident Masdar's eco-friendly design will prove worth the cost.
(Adapted from Unlock)