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.