A:is raining; must have gone
B:rained; would go
C:was raining; should have gone
D:have rained; could have gone
俄罗斯东部地区有许多大河,但河运价值不高,其主要原因是( )①河流结冰期较长 ②河流流向与主要货流方向不一致③河流沿岸人烟稀少,无城市 ④内河航运量小,运费高
车轮周长一定,所行驶的路程和车轮的转数成____比例.