填空题 A. occurs most infrequently
B. is shifted sideways by strong winds
C. is often hidden from our view
D. is equipped with a good knowledge of various forms of lightning
E. is estimated at 20 millions a year
F. is positively charged

填空题 In most cases of cloud-to-ground lightning, the ground's surface ______.

填空题 One form of lightning that ______ is ball lightning.

填空题 Cloud lightning looks lime a ribbon when its lightning channel ______.

填空题 Although not fully understanding processes of lightning, man ______.