以下叙述中正确的是______。

A．预处理命令行必须位于源文件的开头

B．在源文件的一行上可以有多条预处理命令

C．宏名必须用大写字母表示

D．宏替换不占用程序的运行时间

At that time, China was _____ Japan, so going abroad was extremely difficult.[ ]

A. at the war with                  

B. at the war against

C. at war with                    

D. during the war with