Once the audience had filed out, Serge began to wonder who, if anyone, had listened to his long speech.
A. began to wonder who, if anyone, had listened to his long speech.
B. began wondering whom, if anyone, had listened to his long speech.
C. began wondering whom, if anyone, listens to his long speech.
D. began to wonder who, if anyone, was listening to his long speech.
E. began wondering who, if anyone, was listening to his long speech.

[spoiler]OA:A[/spoiler]

Personally i use subject / object rule to differentiate between who and whom.
On reading the sentence, I feel Serge is the subject, but i got the answer wrong... Please let me know the correct usage of who or whom... thanks..