The correct answer is A. We practiced for the debate and many arguments were rehearsed.
As you can see in this sentence, in the first part the pronoun <em>we </em>is used, and in the latter part, that pronoun just disappears and the sentence suddenly becomes passive rather than active. The shift isn't done properly - it is rather awkward. The other options are consistent - they use only one voice rather than two (like in sentence A).
