He made a wonderful discovery, ___ of great importance to science.
| A．which I think is | B．which I think it is | C．which I think it | D．I think which is |
In no country other than Britain, it has been said, ____ experience four seasons in the course of a single day.
| A．people can | B．you can | C．can it | D．can one |
They have no idea at all ____.
| A．what he is working on | B．what is he working on |
| C．what did he work on | D．what he is working |
________ on the top of the mountain is an ancient tower dating back to hundreds of years ago .
| A．To stand | B．Having stood | C．Standing | D．Stand |
No sooner _____ themselves in their seats in the theatre _______ the curtain went up.
| A．they have settled; before | B．had they settled; than |
| C．have they settled; when | D．they had settled;than |
Only when _______ .
| A．did he arrive the meeting began | B．he arrived did the meeting begin |
| C．did he arrive the meeting begin | D．he arrived the meeting began |