What is valve overlap?

A、Thetimebetweenairinletvalveclosingandtheexhaustvalveclosing.
B、Thetimebetweenairoutletvalveopeningandtheexhaustvalveclosing.
C、Thetimebetweenairinletvalveopeningandtheexhaustvalveopening.
D、Thetimebetweenairinletvalveopeningandtheexhaustvalveclosing.