We can never expect bluer sky unless we create less polluted world.
| A．a; a | B．a; the | C．the; a | D．the; the |
After a knock at the door, the child heard his mother’s voice ______ him.
| A．calling | B．called | C．being called | D．to call |
Pop music is such an important part of society ______ it has even influenced our language.
| A．as | B．that | C．which | D．where |
With the help of high technology, more and more new substances ______ in the past years.
| A．discovered | B．have discovered |
| C．had been discovered | D．have been discovered |
--- Guess what! I have got A for my term paper.
--- Great! You ______ read widely and put a lot of work into it.
| A．must | B．should | C．must have | D．should have |
--- Did you tidy your room?
--- No, I was going to tidy my room but I ______ visitors.
| A．had | B．have | C．have had | D．will have |