英语难题!关于定语从句的从属连词
下面是我在资料上摘抄的习题：(括号中的为正确答案）
Is this the river (in which) we can swim?为什么不用which?
I love places( which )the people are friendly.为什么不用where?
I will never forget the days (which）we spent in Beijing last summer.为什么不用when?
请就上面的例句清晰地讲解which的用法