若有定义: float w;  int a, b; 则合法的switch语句是

     A) switch(w)                      B) switch(a);

        { case 1.0: printf("*\n");        { case 1 printf("*\n");

          case 2.0: printf("**\n");         case 2 printf("**\n");

        }                                 }

     C) switch(b)                      D) switch(a+b);

        { case 1:   printf("*\n");        { case 1:  printf("*\n");

          default:  printf("\n");           case 2:  printf("**\n");

          case 1+2: printf("**\n");         default: printf("\n");

        }                                 }