[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。
[完成句子]The importance of particular metals in the human diet has been realized within the past few decades， and the idea that geology might be related to health has been recognized for a number of elements such as iodine， zinc and selenium. For example， soils with low iodine contents produce crops， and animals deficient in iodine. A lack of iodine in the human diet leads to some serious diseases.
Some serious diseases is connected with deficiency of __________.
A、industrially polluted soils
B、the element of iodine
C、rock and soil chemistry
D、the persistence of heavy metals
答案解析：通过关键词Some serious diseases，可将题目定位于最后一句话：A lack of iodine in the human diet leads to some serious diseases. deficiency of和lack of表达同样的意思“缺乏、缺少”，因此根据这句话可以判断答案为B。