若有以下变量定义:float x; int a,b;则正确的switch语句是
A. switch(x){ case 1.0:printf("*\n");case 2.0:printf("* *\n");}
B. switch(x){ case 1,2:printf("*\n");case 3:printf("* *\n");}
C. switch(a+b){ case 1:printf("*\n");case 2*a:printf("* *\n");}
D. switch(a+b){ case 1:printf("*\n");case 1+2:printf("* *\n");}