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At last firefighters have put out a big forest fire in California．Since then，they   (set) about finding out  the fire began．Forest fires are often caused by   (break)glass or by cigarette ends which people carelessly throw away．Yesterday the   (fireman)examined the ground but were not able to find any．They were also quite sure that a cigarette end didn’t start the fire．However，several hours passed    a fireman accidentally discovered the cause．He noticed the remains of a snake which was wound(蜿蜒；缠绕)around the electric wires of a 16，000-volt power line．In this way，he was able to solve  mystery．The   (explain) was simple but very (usual)．A bird had snatched up the snake from the ground and then (drop)it on to the wires．The snake then wound   round the wires．When it did so，it sent sparks down to the ground and these immediately started a fire．