OG 13 35 vs. 72: when vs. in which

skysworder

在35题里，OG的解释是选项D用in which是不对的，可是72题里when和in which都是可以的。是因为在这两个句子里修饰的对象不一样吗？求助！

35题D：earned at a time in which avitation was still so new...

72题A：propose the concept of an age in which great ice sheets had existed...

ukyo2003

in which任何时候都可以用，when需要用在明确时间点，我觉得那个age when的是错的吧，我的理解是age不是明确时间点，只能用in which。

skysworder

又仔细想了一下，at a time本身的意思应该是during any one period，所以表示的是一段时间，而in强调的是一个时间点，所以35题或许可以用during which，但不可以用in which。

skysworder

但是曼哈顿上的解释是：The pronoun when can be used to modify a noun event or time, such as period, age, 1987, or decade. In these circumstances, you can also use in which instead of when. 似乎in which和when始终是可以互换的。迷茫了……