19. Which of the following descriptions is not correct ? (A) Attenuation causes the length of the RNA molecules to be significantly shortened. (B) Arabinose can derepress the ara operon by causing AraC to loosen its attachment to araO2 and to bind to araI2 instead. (C) Recent experimental evidence has shown that three lac operator sequences are needed for maximal expression. (D) A riboswitch is usually a region in DNA that contains an aptamer and an expression platform. (E) The two dimmers in a lac repressor tetramer interact with the major groove of the DNA.