31. A pair of primary and secondary antibodies (Ab) are generally used in a Western blotting experiment. These two antibodies have to come from different sources (e.g., mouse and rabbit), because (A) Ab from different sources will not compete for the same target. (B) these two Ab are modified differently. (C) the secondary Ab recognizes the Fc region of the primary Ab. (D) the primary Ab is more specific than the secondary Ab. (E) Ab from the same source will aggregate.