If we write a recursive function and find that it's slow, but then we speed

it up using tail recursion, what changed to make it go faster?

A: It's avoiding doing redundant work multiple times by remembering

intermediate results.

B: The compiler rewrote the program with a loop, so it's now roughly as fast

as using iteration.

C: We were doing two recursive calls before, which caused an exponential

slowdown $--$ now we're only doing one recursive call.

D: Now our variables are allocated on the heap, which makes the program go

faster.