嗯 选A 因为这里是定语从句
everything seems very hard修饰 periods,意为“每个人在生活中都有事事艰难的时候”
这里periods 做状语,又表时间,所以填when
正常语序为：everything seems very hard (during periods) in their time.
再举个例子,供你揣摩啊：
We will never forget the days when we lived in the countryside,
我们永远不会忘记住在乡下的那些日子.
(We lived in the countryside in these days.)