Adele has reached another milestone with her smash album, 21.
Billboard reports 21 has reached the 10-million-sales mark in the U.S. after less than two years. Coincidentally, it's also the 21st album to hit the milestone since Nielsen SoundScan began tracking such figures in 1991.
Two other albums reached 10 million sales this year: Confessions by Usher and Hybrid Theory by the rock band Linkin Park .
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