He was thinking about the problem when an apple fell off the tree, _______ him right on the head.
| A．hit | B．having hit | C．hitting | D．and hitting |
How this kind of plant can be ____ in a special way is still unknown to us.
| A．kept away | B．kept out | C．kept alive | D．kept off |
If I ___ free tonight, I'll go to the cinema with you.
| A．am | B．was | C．will be | D．be |
Edison had built a chemical lab of ___ when he was very young.
| A．himself | B．him | C．itself | D．his own |
This is the only one of the books on space ___.
| A．that has been written | B．that have been written | C．which have been written | D．which has been written |
---Please help me, will you? ---_____.
| A．You’re welcome | B．Of course, I will | C．Not at all | D．It’s my pleasure |