A:外心
B:内心
C:重心
D:垂心
_________ only 20 minutes to go before the train left, I felt uneasy in the taxi to the railway station.
China has ___________ population in the world.
125×5____125÷5
515÷5____505÷5
840÷4____480÷4
420÷6____420÷2÷3
315÷3____720÷4
360÷3×2____360÷3÷2
乌拉尔山以东依次排列的地形区是( )