20. (A)after (B)when (C)because (D)so that

29. Generally speaking, a moderator does not participate in the discussion unless he wants to. What does this sentence mean? (A)A moderator can never attend the discussion. (B)A moderator has to attend the discussion unless he is a general. (C)A moderator can attend the discussion if he wants to. (D)No moderator can speak in the discussion unless he wants to.