20. Write the DOS command to erase a file named FILEBILL which is stored in the sub-directory named MYFILES located at the root of drive C. 21. Write the DOS command to return you to the root of drive C if your current directory is C:\TEACHING OPSYS\SUMM 22. Write the DOS command to erase all the files on drive A. 23. Write the DOS command to delete a subdirectory named DIRFRED which is located at the root of drive C. 24. Write the DOS command to make the COMMAND.COM file Read Only. 25. Write the DOS command to copy all the files from a directory named DIRECT1 as well as files in its sub-directories? 26. Write the DOS command to view all the hidden files in the current directory 27. Write the DOS command to launch the DOS editor to edit a file named FRED which is located on drive A. 28. Write the Pos command to execute a batch command named BACKUP. 29. Write the command to place a remarks comment in a batch file for the purpose of internal documentation 30. Write the combination of keystrokes that will interrupt the execution of any batch file