In old times，man considered lightning to be one of the great mysteries（神秘事物）of nature．In fact a single flash of lightning 1.6 kilometers long has enough electricity to light one million lamps．
    The American scientist，Benjamin Franklin，was the first to suggest that lightning is in fact electricity formed between the earth and clouds high above．It happened in 1752．In the same year，he also built the first lightning rod（避雷针）to protect buildings from being struck by lightnings．
    Modern science has discovered that lightning is very strong and powerful．A lightning between a cloud and the earth may be as long as 13 kilometers，and travels at a speed of 30 million meters per second．Scientists have the opinion that there are about 2000 million flashes of lightning per year．In the United States alone，it kills almost one person per day．
    The safest place to be in during an electrical（电的）storm is a closed car．Outside，one should go close to low ground for cover and not stop under a tree．Also one should stay out of water and away from metal fences．Inside a house people should keep away from open doorways and windows and not touch wires or metal things．

本初子午线是指（     ）

为下列句子选择恰当的词语，将字母填在横线上。

 长江很快便醒过来了。它以鱼肚色________着缕缕玫瑰红的曙天。

人体必需的“第七类营养素”是指____．