单选题 Google's Gmail system, which serves millions of customers around the world, shut down yesterday. The problem may hurt its efforts to market applications which include e—mail to businesses. Google prides itself on the reliability of its software, at least according to its marketing pitch. It is not clear why Google had the problem. It keeps thousands of servers. Some of those may have developed trouble or some outside programmer, a hackea may have sent a bug into the system to make it collapse.
What happened to Google yesterday?

• A. It lost millions of customers.
• B. Its Gmail system collapsed.
• C. Its market application program failed.
• D. Its servers were destroyed by hackers.