单选题AThe causes are obvious.BThe causes are very complicated.CThe causes are familiar.DThe causes are not well understood.
单选题
A
The causes are obvious.
B
The causes are very complicated.
C
The causes are familiar.
D
The causes are not well understood.
参考解析
解析：
推断题。从录音开头部分“their cause and control remain a serious problem that is difficult to solve”可推断，由于原因复杂，所以难以解决。故选B。
推断题。从录音开头部分“their cause and control remain a serious problem that is difficult to solve”可推断，由于原因复杂，所以难以解决。故选B。
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