Can someone please explain why answer (C) is wrong?
I understand why answer (A) is the correct answer, but answer (C) sounds right to me too.
1. In the first sentence of the stimulus, the author claims that humorous ads are the only effect ads. This seems to mean two things:
A. If an ad is humorous, then the ad must be effective
effective ads

humorous
B. Humorous ads are effective
humorous
effective ads
2. In the last sentence of the stimulus, the author claims effective ads must convey its message:
effective ads
convey message
3. In the second sentence of the stimulus, the author claims that humorous ads can "hold their attention long enough for a message to be conveyed"; I think we can tell from the phrase "hold their attention long enough for a message to be conveyed" that "hold their attention long enough" is a necessary condition for "conveying a message"(the first post in this thread read it this way too):
convey message
hold their attention long enough
4. Combining the conditional relationships in 2. and 3.:
effective ads
convey message
hold their attention long enough
5. From the second sentence of the stimulus, we know that humorous ads possess the ability to hold their attention long enough:
humorous
hold their attention long enough
6. Now we refer back to the conditional relationship in 1. B.:
humorous
effective ads
So the author seems to mistaken a necessary condition for an ad to be effective as a sufficient condition for an ad to be effective. Otherwise, how can the author conclude that humorous ads are effective?