A.txt中为应有文件，B.txt中为实有文件（少于A中文件个数），检测A中缺少的文件,将缺少的文件名输出到C.txt中。-A.txt lists the files names which are should be included, B.txt lists the file names which are actually included(The files number in B.txt is less than that in A.txt ).Checking the lack fi