The boys followed the marks that they ________, until finally they found their way back to the campsite

At the foot of Wulian Mountains, you can hear the _______ of the running water.

It isn’t quite whether the teacher will collect the homework by Friday, so you needn’t hurry.

In my point of view, the question is not _____ the world is going to have a new economic crisis,but when.[ ]

Do you mean to play the hero in the film— _____. But it should be a character I like.