A:has opened; haven't gone
B:has been open; haven't been
C:has been open; haven't gone
D:has been opened; haven't been
____
在一个生态系统中, A种群有10个个体, B种群有100个个体, C种群有1000个个体, D种群有10000个个体,则它们的食物链关系是( )
鸟类有适应飞翔的特征:身体大多是____,前肢变成翼,体表被覆____.
只将一个“220V 800W”的电暖气接入示数为如图所示的电能表所在的电路中,正常工作5h,消耗电能____ kW•h,此时电能表示数变为____