Section B
Directions: There are 2 passages in this section. Each passage is followed by some questions or unfinished statements. For each of them there are four choices marked A, B, C and D. You should decide on the best choice.
Drought occurs when moistures disappear through evaporation faster than it is replaced by rain. A drought may happen in any climate zone from tropical to tundra (土地带). Some of the worst drought of this century occurred in the mid-1980s in the sahel region of West Africa when the summer rains failed.
Water shortages can also affect the countries of Europe as happened in 1976 when Britain faced the worst drought in its history, coupled with temperatures rising up past 30℃.
One of the greatest droughts of modern times occurred in the Midwest United States between 1934 and 1941. The causes of the disaster were not entirely natural as unwise fanning practices also played a part. During the First World War, farmers expanded their fields to take advantage of soaring wheat prices.
After the war, prices fell again, however, and large numbers of cattle were turned into the ploughed fields. Millions of hooves(蹄) crumbled the soil and the winter winds blew it away. From 1934 drought conditions made matters worse. Dust lifted up from the land in 300-metre high clouds that swept over farms and suffocated animals in storms that lasted for days. In the summer, desert heat baked the land in conditions that were too much even for rattlesnakes to survive. Nineteen states felt the effects of the drought.
"It" in the first sentence of this passage refers to ______.

A. drought
B. moisture
C. evaporation
D. rain