单选题

　　1.【完成句子】The average lightning strike is more than 3 miles long and can travel at a tenth of the speed of light. Ball lightning，the rarest and most mysterious form，derives its name from the small luminous ball that appears near the impact point，moves horizontally，and lasts for several seconds.

　　One form of lightning that __________ is ball lightning.

　　A.occurs most infrequently

　　B.is shifted sideways by strong winds

　　C.is often hidden from our view

　　D.is estimated at 20 millions a year