By 2000, Napster had seen mainstream adoption, and several music publishers responded by starting to sell some CDs with various copy protection schemes. Most of these were playback restrictions that aimed to make the CD unusable in computers with CD-ROM drives, leaving only dedicated audio CD players for playback. This did not, however, prevent such a CD from being copied via an analogue connection or by ripping the CD under operating systems such as Linux, which was effective since copy-protection software was generally written for Microsoft Windows. These weaknesses led critics to question the usefulness of such protection.