20. 以下正确的函数形式是( )
A. double fun(int x,int y) {z=x+y; return z;}
B. fun(int x,y) {int z; return z;}
C. fun(x,y) {int x,y; double z; z=x+y; return z;}
D. double fun(int x,int y) {double z; z=x+y; return z;}
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