Which answer is a correctly punctuated compound sentence?

A.
Helen's left front tire is losing air and the mechanic thinks she may need a new tire.

B.
Helen's left front tire is losing air, the mechanic thinks she may need a new tire.

C.
Helen's left front tire is losing air the mechanic thinks she may need a new tire.

D.
Helen's left front tire is losing air; the mechanic thinks she may need a new tire.