Question

A runner completes a more difficult run than usual. At the end, he's breathing heavily and his muscles are cramping. This condition is caused by a buildup of ___ in his blood and muscles.

A. lactic acid
B. oxygen
C. carbon dioxide
D. sulfur

I think it's lactic acid but i want to make sure.
bobpursley
Go to the head of the class, you are right.
Collin
That's right and thanks for answering my question.