[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
答案解析：本段文字与题目相关的句子是：Ball lightning, the rarest and most mysterious form, derives its name from the small luminous ball…，关键词是rarest(rare的最高级形式)与infrequently“很少发生，少有”表达相同的意思。
[概括大意]Wignall calculated the “killing efficiency” for these volcanoes by comparing the proportion of life they killed off with the volume of lava that they produced. He found that size for size， older eruptions were at least 10 times as effective at wiping out life as their more recent rivals.
The main idea of this paragraph is__________.
A、Killing Power of Ancient Volcanic Eruptions
B、Accounting for the Killing Power of Older Eruptions
C、Calculation of the Killing Power of Older Eruptions
D、A Mass Extinction
答案解析：本段第一句话是说：Wignall calculated the "killing efficiency" for these volcanoes…;Wignall 计算了这些火山的“灭绝效率”……;其中killing efficiency与killing Power意思差不多，属于同义词转换，因此选择B或C，但是account for 意思是：(在数量方面)占;说明(原因等)不符合句意，因此不选择B。