Trafficlightsarered,yellowandgreen.Theyareveryimportant...
问题详情：Trafficlightsarered,yellowandgreen.Theyareveryimportantsignals(信号).TrafficlightsareusuallyplacedatbusystreetcornerstohelpcontroltrafficSometimes,theyareonstreetsnearschoolsandshops,too.Mosty...