请为下面的句子填入正确的介词。 1. Those days when I studied at college were full of fun. = Those days ______ which I studied at college were full of fun. 2. The table where your mother put the book is very beautiful. = The table ______ which your mother put the book is very beautiful. 3. The reason why he was absent is obviously ridiculous. = The reason ______ which he was absent is obviously ridiculous.