单选题
According to the passage, what helps to explain why the population problem has come on "all of a sudden"?
【正确答案】
C
【答案解析】[试题分析] 细节题。
[详细解答] 文章第二段第一句谈到This helps explain why the population has come on“all of a sudden”，而根据上下文，this指代前文关于硬币的例子。因此，本题答案为C。