Air pollution is getting more and more serious, so we must take action __________ it is too late.
| A．before | B．after |
| C．until | D．when |
He has three brothers, ________is a doctor.
| A．all of them | B．all of who |
| C．none of whom | D．both of them |
As teenagers of the 21st century, we should be ready to _____ the challenge of society.
| A．put | B．do |
| C．make | D．meet |
The reason _____ they held a party was _____ it was Sue’s birthday.
| A．why; because | B．why; that |
| C．that; that | D．that; because |
--Why do you look pale today?
--I was very ________my mother’s illness. I stayed awake last night.
| A．concerned about | B．concerned with |
| C．concerned in | D．concerned to |
I _____ in the street when someone _____ me.
| A．walk; called | B．walking; calling |
| C．was walking; called | D．walked; called |