“襄樊大道”是襄阳市为纪念襄樊这个曾经的历史名称而命名的又一条景观大道，全长约6千米，如果公交汽车在“襄樊大道”上平均的行驶速度是30千米/小时，那么公交汽车跑完全程需要的时间是____ min．

Liu Tao: What's the time now, Yang Ling?

Yang Ling: It's four thirty. It's time to go home.

Liu Tao: It's raining(下雨).Where is my umbrella?

Yang Ling:Look!There's a red umbrella under the desk.

Liu Tao:That isn't my umbrella.

Yang Ling:There are four umbrellas near the blackboard. Which one is yours（你的），Liu Tao?

Liu Tao:The blue one. Thank you, Yang Ling.

Yang Ling:Shall we go home together？

Liu Tao:Great!Let's go.

按气候统计标准划分的四季，北半球的春季指（　　）