since 是因为的意思。如果非要加上since ,那就在句子的后面说个原因。 学习英语请不要钻牛角尖。
现在的英语把学生都考傻了。哎。
也就是说since 虽然是完成时的一个标志,但是要具体情况具体分析。如果这样的一句话就要用since :She has been to Beijing to pursue her dream since she was 21 years old. 如果这样的一句话就要用before: She was told have to be 21years old before she can go to Beijing to pursue her dream.
如果在不明白,你学英语就不要理解着学,你就背课文吧,我上学的时候可是一篇课文都不差全背的。最后虽然解释不明白是怎么回事,但是我就是写的对。这个就是传说中的语感。
before除了有“在...以前,之前”的意思外,还有"...才能..."的意思,在这句话中,理解为"...才能...",故选B。
如果是A就必须是过去时。
比如:It has past 3 weeks since he recorvered and reterned to work.
强调在他工作前,至少有3周。
he is told that It would have been at least 3 weeks since he could recover and reture to work.
since 有自从和因为两个意思,在这里都不适合句意