以下正确的函数形式是(　)Adoublefun(intx，inty){z=x+y;returnz;}Bfun(intx，y){intz;returnz;}Cfun(x，y){intx，y;doublez;z=x+y;returnz;}Ddoublefun(intx，inty){doublez;z=x+y;returnz;}