Why doesn't free() zero out the memory prior to releasing it?

Question

When we free() memory in C, why is that memory not filled with zero? Is there a good way to ensure this happens as a matter of course when calling free()<

Answer 1

C why is the memory not explictly set to zero in the free implementation .

Because of speed.

Because after we free the memory any how we set it to zero after freeing.

Eh?