种植农作物要合理密植,原因是( )
A:增加植株周围的二氧化碳浓度
B:增加吸收光能的作物叶片的面积
C:降低周围空气中氧气的浓度
D:降低周围的气温以增加有机物合成
In my driving lesson, I learned a traffic rule that at no time _____ parked in a manner which will block emergency service.
在△ABC中,A:B:C=1:2:3,则a:b:c等于( )