答案:[解析]“It's… before”句型表示“(……之后)才……”之意,如: It was several weeks before I got a decent night's rest.几星期以后我才得以好好地睡了一夜。故A符合题意。until意为“直到……为止”,在否定句中多译为“直到……才”,该词一般不用在“it is…until”(肯定)句型中, 如: They talked on until one o'clock In the morning.他们一直谈到凌晨一点钟。“It is(has been)… since”句型表示“自……以来已有……时间了”,例如: It has been only twenty five years since television came to control American free time.电视开始主宰美国人的空闲时间,至今也不过才25年。when通常表示“在…… 时”,用在本题不符合题意。