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NEW YORK/SAN FRANCISCO (Reuters) - Apple Inc shares tumbled more than 6 percent on Wednesday, chalking up their biggest single-day loss in four years as fears grow about intensifying competition in the mobile device market.
Investors and analysts blamed the sell-off on a mix of factors, including a forecast by an influential research firm that the iPad maker is continuing to cede ground to rival Google Inc's Android gadgets, and unconfirmed reports that at least one major stock-clearing house was raising margin requirements on Apple stock trades.
