It was a village in India. The people were poor. However, they were not unhappy. After all, their forefathers had lived in the same way for centuries.Then one day. Some visitors from another city arrived. They told the villagers there were some people elsewhere who liked to eat frog’ s legs. However, they did not have enough frogs of their own, and so they wanted to buy frogs from other place.This seemed like money for nothing. There were millions of frogs in the fields around, and they were no use to the villagers. All they had to do was to catch them. Agreement was reached, and the children were sent into the fields to catch frogs. Every week a truck arrived to collect the catch and hand over the money. For the first time, the people were able to dream of a better future. But the dream didn’t last long.The change was hardly noticed at first, but it seemed as if the crops were not doing so well. More worrying was that the children fell ill more often, and, there seemed to be more insects around lately.The villagers decided that they couldn’ t just wait to see the crops failing and the children get-ting weak. They would have to use the money earned to buy pesticides (杀虫剂) and medicines. Soon there was no money left.Then the people realized what was happening. It was the frog. They hadn’ t been useless. They-had been doing an important job—eating insects. Now with so many frogs killed, the insects were increasing more rapidly. They were damaging the crops and spreading diseases.Now, the people are still poor. But in the evenings they sit in the village square and listen to sounds of insects and frogs. These sounds of the night now have a much deeper meaning. What might be the cause of the children’ s sickness().

A. The crops didn’t do well.
B. There were too many insects.
C. The visitors brought in diseases.
D. The pesticides were overused.