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Apple's iTunes grows to No. 2 U.S. music retailer
Based on 2007 sales, legal music downloads now account for 10 percent of music acquired in the United States, but the increase has not offset a decline in physical CD sales, NPD said.
The iTunes store now has more than 50 million customers and has sold more than 4 billion songs, Apple said in a separate statement.
(Reporting by Michele Gershberg; Editing by Lisa Von Ahn)