The Space Shuttle Columbia broke into pieces over Texas as it returned to the earth on February 1，2003，__________all seven astronauts aboard.
| A．having killed | B．killing |
| C．being killed | D．killed |
We didn’t go home _____ we finished the work.
| A．since | B．until | C．because | D．though |
I'll stay here _____ everyone else comes back.
| A．even if | B．as though | C．because | D．until |
Although it's raining, _____ are still working in the field.
| A．they | B．but they | C．and they | D．so they |
Speak to him slowly _____ he may understand you better.
| A．since | B．so that | C．for | D．because |
You'll miss the train ______ you hurry up.
| A．unless | B．as | C．if | D．until |