【正确答案】 B

【答案解析】[试题分析] 本题考查对文章事实的理解。
[详细解答] 第五段第三句Millions of people between the latitudes of Cairo and the Cape of Good Hope were subjected to a choking atmosphere that grew damper and hotter from day to day，已指出了原因是atmosphere that grew damper and hotter from day to day，而这正是前文the humidity of the surface waters of the sea increased的结果，所以，正确答案为B。