As you say, task A is to classify the stance of the replies (including the stance of the original post, which is different from the veracity of the proposition contained in it. For example, if I say "I don't believe the sky is blue", task A is to assign "deny" and task B is to assign the truth of the proposition that the sky is blue. Task A also involves classifying all the responses to that of course).
Task B is to classify the veracity of the proposition in the original post.
Hope that helps,
Genevieve