Nobody believed a 92-year-old man could run a marathon_______he passed the finishing line. People of all ages can challenge themselves.

A．if

B．since

C．until

D．unless

执行语句序列
ofstream outfile("DATA．DAT")；
if(．．．)cout＜＜"OK"；else cout＜＜"FAIL"；
后，如果文件打开成功显示"OK"，否则就显示"FAIL"。由此可知，上面if语句的…处的表达式应是

A．outfile．fail()或outfile

B．outfile．good()或!outfile

C．outfile．good()或outfile

D．outfile．fail()或!outfile