若u、w、x、y均是正整型变量，则以下正确的switch语句是（     ）。   A:switch(x*x+y*y){ case 3: case 3: w=x+y; break; case 0: w=y-x; break;}B: switch(pow(x,2)+pow(y,2)){ case 1: case 3: w=x+y; break; case 0: case 5: w=x-y;}C:switch x{ default : u=x+y; case 10 : w=x-y; break; case 11 : u=x*y; break;}D:switch(x+y){ case 10 : u=x+y; break; case 11 : w=x-y; break;}答案: switch(x+y){ case 10 : u=x+y; break; case 11 : w=x-y; break;}