— What's the matter with Andy ? He doesn't look very well.  

— He has a headache because he ______ too long; he ought to have stopped reading.[ ]

A. had read    

B. is reading       

C. was reading    

D. has been reading

有如下程序：
#include＜iostream＞
using namespace std;
class CA
public:
CA()cout+’A’;
;
class CB: private CA
public:
CB()cout＜＜’B’;
;
int main()
CA a;
CB b;
return 0;

这个程序的输出结果是______。