We’ll have to race _____ the clock to complete the work on time.

A．beyond

B．over

C．from

D．against

有以下程序：
#include ＜iostream＞
using namespace std;
class sampleprivate:
int n;
public:
sample()
sample(int m)

n=m;

sample add(sample s1,sample s2)

this-＞n-s1.n+s2.n;
return (*this);
void disp()

cout ＜＜"n="＜＜n＜＜end1;
;
int main ( sample s1(10),s2(5),s3;
s3.add(s1,s2);
s3.disp();
return 0;程序运行后，输出的结果是

A．n=10

B．n=5

C．n=20

D．n=15