Given the declaration: float w; int a, b; , which one of the following is a legal switch statement?

A．switch (a);
       {      case 1:printf("*\n");
              case 2:printf("**\n");
       }
B．switch (b)
       {      case 1:printf("*\n");
             default:printf("\n");
              case 1+2:printf("**\n");
       }
C．switch (a+b);
       {      case 1:printf("*\n");
              case 2:printf("**\n");
              default:printf("\n");
       }
D．switch (w)
       {      case 1.0:printf("*\n");
              case 2.0:printf("**\n");
       }