新概念英语第53课Hot Snake.A bird had snatched up the snake from the ground.为什么要用过去完成

新概念英语第53课Hot Snake.A bird had snatched up the snake from the ground.为什么要用过去完成
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At last firemen have put out a big forest fire in California.Since then,they have been trying to find out how the fire began.Forest fires are often caused by broken glass or by cigarette ends which people carelessly throw away.Yesterday the firemen examined the ground carefully,but were not able to find any broken glass.They were also quite sure that a cigarette end did not start the fire.This morning,however,a firemen accidentally discovered the cause.He noticed the remains of a snake which was wound round the electric wires of a 16,000-volt power line.In this way,he was able to solve the mystery.The explanation was simple but very unusual.A bird had snatched up the snake from the ground and then dropped it on to the wires.The snake then wound itself round the wires.When it did so,it sent sparks down to the ground and these immediately started a fire.