They were surprised that a child should work out the problem____ they themselves could not.
| A．once | B．unless | C．while | D．if |
Our professor told us that salt water _____ at a lower temperature than fresh water.
| A．is freezing | B．was frozen | C．freezes | D．froze |
Until recently historians assumed that calendars ________ with the advent of agriculture.
| A．came to existence | B．came to existing |
| C．came to exist | D．were existential |
. ________ when the boulders suddenly disappeared.
| A．So relieved we felt | B．How relieved we felt |
| C．What a relief we were feeling | D．How we felt relieved |
________ from endless homework on weekends, the students now find their own activities, such as taking a ride together to watch the sun rise, more colourful.
| A．Freed | B．Freeing | C．To free | D．Having freed |
When he knew the disappointing result of the exam, he seemed quite _______.
| A．fed up | B．cast down | C．in vain | D．in amazement |