动脉血和静脉血的区分标准是( )
A:氧气含量的高低
B:是在动脉血管还是在静脉血管内的流动
C:营养物质的多少
D:没有区分标准
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具有“沟壑纵横”的地形特点的高原是( )
304+336=____
684+291=____
381+319=____