Lightening and thunder seem very different from fog，clouds，rain，wind，heat，and other things which make up the weather.____11____ without knowing about the rest of the ____12____，one cannot understand lightening and thunder. Long ago，____13____ scientists had begun to study the weather，everyone was afraid of lightening and thunder. ____14____ the ancient Greeks and Romans，thunder was the angry voice of ____15____ great god Zeus，or Jupiter. Today we know more about weather and ____16____ lightening and thunder too，though there ____17____ still much to be learneD、People today are not often afraid of thunderstorms，though we know that lightening ____18____ dangerous. ____19____ we understand lightening and thunder，there is a thrill (激动) in watching a summer thunderstorm which clears the air and leaves us ____20____.