Chọn một phương án A, B, C, hoặc D để điền vào chỗ trống trong văn bản sau. 
Overcrowding occurs when there are so many people living in a small area. This (17)______ many problems to the people who live there. Take Mumbai, a big city in India, for example. The city has a (18)______of 21,000 people per square kilometer. When a city becomes overcrowded, various problems might happen. One problem is there is not enough space for evryone to live comfortably. Heavy traffic congestion is another problem. The streets become so crowded with cars, buses and motorbikes, (19)______ it is difficult for people to get around. This can make travel times longer and people feel annoyed. Also overcrowding can have a negative impact on the city's resources, including water, electricty, (20) ______healthcare facilities. Overall, overcrowding is s significant problem that presents serious challenges requiring effective solutions. 
17. A. causes          B. makes            C. takes                 D. does 
18. A. populate      B. populous      C. population       D. populated 
19. A. who              B. when             C. whose               D. which 
20. A. but                B. and                C. or                       D. so