It's a waste of time ( to do - doing ) all these silly exercises.

Question

It's a waste of time ( to do - doing ) all these silly exercises.

Answer 1

to do, doing, to be doing

a lot of to do about nothing, a really silly exercise. Tell your instructor or text book author that

ABC's of writing favor Accuracy, Brevity, Clarity/Concise. "doing" is briefest, so it's ahead in the grammar horse race odds on favorite