We must do something to ________ the factories sending out poisonous gases so as to keep the city clean and beautiful.
| A．keep | B．protect | C．permit | D．prevent |
19. It’s dangerous to have the fire _______ at home all night long.
| A．to burn | B．burn | C．burned | D．Burning |
18. I decided to write about Chaplin, one of ______ films I had seen several years before.
| A．whose | B．whom | C．that | D．which |
17. Our attention was drawn to the eight-year-old girl who was _______ perfectly at the piano.
| A．acting | B．behaving | C．performing | D．doing |
16. He made another wonderful discovery, __________ of great importance to science.
| A．I think which is | B．which I think it is | C．which I think it | D．which I think is |
15. Great changes ______ in the city, and a lot of new buildings ______.
| A．have been taken place; have been set up | B．have taken place; have been set up |
| C．have taken place; have set up | D．were taken place; were set up |