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
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。
1.【完成句子】Heavy metals are persistent;they do not break down to other chemicals in the environment. Industrially polluted sites usually undergo intensive clean-up and rehabilitation because heavy metals are a health concern once they enter the food chain. Some trace metals are alleged to cause cancer and are also known to cause poisoning.
Industrially contaminated sites usually require a thorough clean-up due to __________.
A.the persistence of heavy metals
B.industrially polluted soils
C.naturally polluted soils
D.rock and soil chemistry