- 問題詳情:Imaginearoadcrossingwithoutthethree-waytrafficsignal.Cars,trucks,schoolbuses,bicyclistsandstudentswalkingtoschoolwouldtrytorushthroughthecrossing61_______thesametime.Theirsafetywouldbeindanger.ThankstoanAfricanAmerican62________(invent)namedG...
- 16809