有如下程序： #include ＜iostream.h＞ using namespace std; class Stack { public: Stack(unsigned n=10):size(n){rep_=new int [size]; top=0;} Stack(Stack&s): size (s.size) {rep_=new int[size];for(int i=0;i＜size;i++) rep_[i]=s.rep_[i];top=s.top; }～Stack(){delete[]rep_;} void push(int a){rep_[top]=a; top++;} int pop(){--top; return rep_[top];} bool isEmpty() const {return top ==0;} private: int*rep_; unsigned size, top; }; int main() { Stack s1; for(int i=1;i＜5;i++)sl.push(i); Stack s2(s1); for (int i=1;i＜3;i++)cout＜＜s2.pop()＜＜’,’; s2.push(6); s1.push(7); while(!s2.isEmpty())cout＜＜s2.pop()＜＜’,’; return 0; } 执行上面程序将输出( )。
A．4,3,2,1,
B．4,3,6,7,2,1,
C．4,3,6,2,1,
D．1,2,3,4,