The chairman thought _____ necessary to invite Professor Smith to speak at the meeting.
| A．those | B．that | C．it | D．this |
I hope all the precautions against air pollution, _____ suggested by the localgovernment, will be seriously considered here.
| A．while | B．since | C．after | D．as |
You see the lightning _____ it happens, but you hear the thunder later.
| A．the instant | B．for an instant | C．on the instant | D．in an instant |
It is useful to be able to predict the extent _____ a price change willaffect supply and demand.
| A．fromwhich | B．withwhich | C．towhich | D．for which |
I have kept that portrait _____ I can see it every day, as it always reminds me of my university days in London.
| A．which | B．where | C．whether | D．when |
The ability to store knowledge makes computers different from every other machine _____ invented.
| A．ever | B．thus | C．yet | D．as |