His sister left home in 1998，and __________ since.
| A．had not been heard of | B．has not been heard of |
| C．had not heard of | D．has not heard of |
She was so _____ in her job that she didn’t hear anybody knocking at the door.
| A．devoted | B．attracted | C．drawn | D．absorbed |
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 |