Experts say walking is one of the best ways for a person to _______ healthy.
| A．maintain | B．stay | C．live | D．preserve |
From time to time a crocodile has snatched someone _______ he or she is even aware that the crocodile is there.
| A．when | B．until | C．since | D．before |
_____ unemployment and crime are high, it can be assumed that the latter is due to the former.
| A．Before | B．Where | C．Unless | D．Until |
_____the Internet is of great help, I don’t think it’s a good idea to spend too much time on it.
| A．If | B．While | C．Because | D．As |
Robert is said ___________ abroad, but I don’t know what country he studied in .
| A．to have studied | B．to be studying | C．to study | D．to have been studying |
He made another wonderful discovery, ______ of great importance to science.
| A．which I think is | B．which I think it is |
| C．which I think | D．I think which is |