有如下程序：#include#includeusing namespace std;class MyString{public;char str[80];MyString(const char*s) {strcpy(str,s);}MyString&operator+=(MyString a){strcat(str,a.str);return*this;}};ostream&operator<<(ostream&s,const MyString&z){return s << z.str}int main(){MyString x( abc )，y( cde )；cout<<(x+=y)<<endl;return 0;}运行这个程序的输出结果是
A．abc
B．cde
C．abcde
D．abccde