单项选择题
Lighteningandthunderseemverydifferentfromfog，clouds，rain，wind，heat，andotherthingswhichmakeuptheweather.____11____withoutknowingabouttherestofthe____12____，onecannotunderstandlighteningandthunder.Longago，____13____scientistshadbeguntostudytheweather，everyonewasafraidoflighteningandthunder.____14____theancientGreeksandRomans，thunderwastheangryvoiceof____15____greatgodZeus，orJupiter.
Todayweknowmoreaboutweatherand____16____lighteningandthundertoo，thoughthere____17____stillmuchtobelearned.Peopletodayarenotoftenafraidofthunderstorms，thoughweknowthatlightening____18____dangerous.____19____weunderstandlighteningandthunder，thereisathrill（激动）inwatchingasummerthunderstormwhichclearstheairandleavesus____20____.
A．before
B．after
C．since
D．when