Which sentence corrects the vague pronoun in the underlined sentence?

Trevor and Martina were supposed to visit Marcie over the weekend to work on a project. However, she forgot, and the project wasn't completed in time. That Monday, they were forced to stay after school to complete it and still got a low grade.

A. However, Marcie forgot, and the project wasn't completed in time.
B. However, the project wasn't completed in time because she forgot.
C. However, he forgot, and the project wasn't completed in time.
D. However, when she forgot, the project wasn't completed in time.