The teacher together with his students ___ down at the desk and having a discussion in the classroom.
| A．is sitting | B．sat | C．are sitting | D．would sit |
—Didn’t you buy a dress yesterday?
—No，__________I liked the colour，I didn’t like its style.
| A．since | B．if | C．when | D．though |
_________ I felt somewhat disappointed and was about to leave，something occurred which attracted my attention.
| A．Unless | B．Until | C．When | D．While |
There is no doubt that he is a good professor__________he is doing fine.
| A．but | B．because | C．so | D．yet |
__________ there are a large number of chemical factories，the air is likely to be polluted.
| A．That | B．Where | C．Once | D．As |
_________，this kind of disease can do great harm to people there.
| A．Occurring where |
| B．It occurs where it is |
| C．Where does it occur |
| D．Wherever it occurs |