The skies are raining big blocks of ice, and experts are trying to learn what’s going on. For the second time in six days, California was attacked by ice balls. The latest happened on early Thursday. A large block of ice dropped into Loma Linda, California. The ice crashed through the metal roof ( 房顶 ) of a fitness center, leaving a hole up to 21⁄2 feet wide. No one was hurt. Two tennis players were playing outside the Drayson Center on Thursday morning. Suddenly they heard a strange sound, said Crawford, an official of the Loma Linda Fire Department. A similar happened last Saturday in Oakland, California. Early in the morning an ice ball dropped into a field at Bushrod Park and made a hole up to 2 feet wide. Luckily, no one in the park was injured. One theory ( 理论 ) is the ice was dropped from planes. But there’s little direct support for that. A few experts have suggested there may be other causes of similar ice falls, perhaps even global ( 全球的 ) warming. Stories about dropping ice go back for centuries. They didn’t begin to receive serious scientific attention until a few years ago, however, when Spain was attacked by a dozen large ice balls in 2000. Scientist Jesus Martinez-Frias in Madrid and his team have collected reports of forty cases around the world since 1999 of puzzling falling ice. He thinks the ice forms in the air by a process similar to the formation of hail ( 冰雹 ) inside storms but without a storm. Martinez-Frias believes global warming is causing the troposphere ( 对流层 ) to rise. This means the roof of the troposphere is forced to a greater height, where it cools more than normal. Thus, the temperature difference between warm and cold air in the troposphere produces strong up-and-down winds that repeat the hail-formation process, without a storm. 32. What took place on Thursday morning? A. A fitness center caught fire. B. A big ice ball fell in Loma Linda. C. There was heavy rain in Loma Linda. D. Two tennis players were hit by an ice ball. 33. What is the similarity between the two s in California? A. No one was injured. B. No building was destroyed. C. Both were seen by Crawford. D. Both caused a large number of holes. 34. What do we learn about dropping ice? A. It is very common around the world. B. It often happens in Spain in recent years. C. It has been carefully studied since 2000. D. It is most likely to be produced by planes. 35. What is the cause of ice falls according to Martinez-Frias? A. Strong winds above the troposphere. B. The formation of hail. C. Global warming. D. Big storms.