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.