Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct word or phrase that best fits each of the numbered blanks from 19 to 24.

There are a lot of disadvantages of living in a big city, and air pollution is among (19) ______ most serious problems. Firstly, air pollution comes from the factories in the city. Big cities attract a lot of both inside and outside investments, (20) ______ the number of factories is increasing quickly. This makes the air extremely (21) ______ by smoke from factories. Secondly, that there are too many means of transportation also makes this problem worse. Exhaust from cars, and motorbikes is one of the factors (22) ______ make the air polluted. To conclude, air pollution, which is expected (23) ______ more and more people get many serious (24) ______ such as lung cancer, tuberculosis and so on, is a big drawback of city life.