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若有定义 float x;int a,b; ,则正确的 switch 语句是( )
A.
switch (a+b); { case 1: printf("*\n"); case 2: printf("**\n"); }
B.
switch(x) { case1.0: printf("*\n"); case2.0: printf("**\n"); }
C.
switch (a+b) { case 1: printf("\n"); case 1+2: printf("**\n"); }
D.
switch(x) { case1, 2: printf("*\n"); case3: printf("**\n"); }