Questions17 ～ 20 are based on the following talk. You now have 20 seconds to read Questions 17 ～ 20.
What is the main idea of the story

A．The local Africans are peace-loving people.
B．Soda bottles are very precious in some remote areas.
C．A trivial thing may sometimes bring about undesirable consequences.
D．Caution must be taken in introducing new technology.