___________ that he was in great danger, Eric walked deeper into the forest．
| A．Not realized | B．Not to realize | C．Not realizing | D．Not to have realized |
The new system needs to be in operation for five months ________ it can be updated.
| A．as | B．while |
| C．before | D．once |
We can't wait any longer ________ we'll miss the plane.
| A．or | B．but |
| C．so | D．yet |
House prices vary from place to place and are usually high ________ there are famous schools.
| A．when | B．since |
| C．where | D．whether |
It will be another 50 years ______ our country is expected to reach the level of a medium developed country.
| A．since | B．before |
| C．that | D．when |
Jerry has promised to keep the secret, so he won't tell anyone ________ asked to.
| A．until | B．now that |
| C．unless | D．even though |