长江上游与黄河中上游河段的共同特点是[ ]
A．流量大
B．有结冰期
C．含沙量大
D．水能资源丰富
答案：D
It's the first time that he has been to Australia, _____? [ ]
A. isn't he
B. hasn't he
C. isn't it
D. hasn't it
诊断小麦穗分化的主要依据是（）
