Identify the correctly punctuated sentence.
a. Michael ​Dorris's book The Broken Cord made readers more aware of the problem of fetal alcohol​ syndrome, one of his adopted sons was diagnosed with this condition.
b. Johnny Cash auditioned for a record​ producer, singing mainly gospel​ tunes, was told to come back with new material.
c. Agent Orange is believed to be the cause of some illnesses suffered by veterans was widely used in chemical warfare in Vietnam.
d. One day​ Nick's roommate brought home a​ ferret; he'd seen it at a pet store and​ couldn't resist it.