Apple Inc. faces significant challenges selling its larger and more powerful iPad Pro to businesses because companies are reluctant to switch software vendors and use an expensive device that lacks specialized business apps, analysts said. The enterprise market, which is how Apple refers to its business customers, represents 10 percent of its $183 billion annual revenue, he said. Apple has at least one client so far: General Electric has given some of its 305,000 employees the option to use Apple devices at work, with 20,000 iPads and 60,000 iPhones now available in their offices.