Scientists say human activity contributes to climate change, but they do not agree on the rate_______ climate change may be developing.
| A．with which | B．with that | C．at which | D．at that |
. It is difficult for us to learn a lesson in life __________ we’ve actually had that lesson.
| A．until | B．after |
| C．since | D．when |
.__________ he had not hurt his leg, John would have won the race.
| A．If | B．Since |
| C．Though | D．When |
.I won’t call you, __________ something unexpected happens.
| A．unless | B．whether |
| C．because | D．while |
. He was told that it would be at least three more months __________ he could recover and return to work.
| A．when | B．before |
| C．since | D．that |
.Most birds find it safe to sleep in the trees, but __________ they have eggs or young chicks, they don’t use a nest.
| A．why | B．how |
| C．unless | D．where |