Im giving brainliest :D Which of the following best describes how the boy is able to flex his muscles?

As the boy bends his arm, the triceps muscles contract and extend.

As the boy bends his forearm, the biceps muscles contract and bunch up.

As the boy flexes his arm muscles, both the biceps and triceps muscles contract at the same time.

As the boy moves his forearm, both the biceps and triceps muscles relax at the same time.