Which level should I mock the dependency when the dependency is complicated?

Now I hava a class A to be tested. When I test a method(called Amethod) of A, Amethod will invoke another method(called Bmethod) of class B. And in this method of B, Bmethod will invoke a method of a interface C. It just likes a dependency tree:

A->B->C

I am using the gmock to do the unit test of Amethod of class A. Which one I should mock, B or C?