6 . 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 cigarette ends which people carelessly throw away. Yesterday the firemen examined the ground carefully, but were not able to find any cigarette end. They were quite sure that a cigarette end did not start the fire. This morning, however, a fireman accidentally _________discovered the cause. He notices the remains of a snake which was wound round the electric wire(电线)of 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 quickly started a fire.