【正确答案】assign Mux = (S = = 0)? A : 'bz;
   assign Mux = (S = = 1)? B : 'bz;
   assign Mux = (S = = 2)? C : 'bz;
   assign Mux = (S = = 3)? D : 'bz;
   合法，描述了一个四路选通器。