He is very humorous, while his brother is rather seriousThat is ___their difference lies

Occasions are quite rare _______ I have the time to spend a day with my kids.

下列各句中，加线的成语使用恰当的一句是(　　)

It has been announced that candidates remain in their seats until all the papers have been collected.

Only if enough evidence is discovered _______ in theexistence of the monster.