Which of the following is the correct way to rewrite the following run-on sentence?
mike joined the army he became more disciplined during the training.

a. mike joined the army and more disciplined during the training.
b. mike joined the army and became more disciplined; during the training.
c. mike joined the army, he became more disciplined during the training.
d. mike joined the army, and he became more disciplined during the training.