While working with MS-DOS which command is used to copy all files with extension .txt into one file named all.txt? A. Copy a: *.txt B. Copy *.txt a: C. Copy *.txt c: D. Copy *.txt all.txt

The correct answer is D. Copy *.txt all.txt.

The *.txt wildcard matches all files with the extension .txt. The Copy command copies files from one location to another. In this case, we are copying all files with the extension .txt from the current directory to a file named all.txt.

The other options are incorrect because they do not specify the destination file. Option A would copy all files with the extension .txt to the drive A:. Option B would copy all files with the extension .txt to the current directory, but it would not give them the name all.txt. Option C would copy all files with the extension .txt to the drive C:.