They have just started their trip to Tibet. They prepared for it for about a month.
= They have just started their trip to Tibet (which/that) they prepared for for about a month.
= They have just started their trip to Tibet for which they prepared for about a month.
注:( )表示可以省略不用;/ 表示可以任选其一。
They have just started their trip to Tibet for which they prepared for about a month.
they have just started their trip which they prepared for it about a month to Tibet.