有一读者问： 有这样一道改错题：
(1)We (A: had been) in Athens for (B: not more than) two days (C: until) it became obvious that we (D: needed) a guide.
答案是C错，应改为when。可我认为用until在意思上、结构上似乎都没有错。请问答案正确吗？
所给答案完全正确。until在逻辑上是讲不通的，怎能说“……待了才两天，直到……为止”呢？ 请注意： 这里的when≠“当”，而=“这时突然”，再如：
(2)We hadn't gone ten miles when Sam wanted to go home.
(3)I was just going out when there was a knock at the door.
请注意这种表“突然”或“出人意外”的when从句乃是一句的重心，不可移至句首。
