Most birds find it safe to sleep in the trees, but __________ they have eggs or
题型：不详难度：来源：
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 |
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答案
| C。 |
解析
| 将四个选项的意思分别套入句中，只有填unlessunless时意思最通顺。为便于理解，句子也可转换成：Most birds find it safe to sleep in the trees, but they don’t use a nest unless they have eggs or young chicks. |
举一反三
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 |
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__________ he had not hurt his leg, John would have won the race. | A．If | B．Since | C．Though | D．When |
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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 |
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Many of them turned a deaf ear to his advice, __________ they knew it to be valuable. | A．as if | B．now that | C．even though | D．so that |
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