下列哪种动物的后肢不可以向前弯？（）
A.海狮
B.海狗
C.海豹
D.海象
参考答案：C
This is the first time that I _____ such an interesting film. [ ]
A. saw
B. see
C. have seen
D. had seen
抢救呼吸窘迫综合征的关键措施是
A．防止感染
B．保持酸碱平衡
C．代谢支持
D．利用利尿剂
E．纠正低氧血症和改善肺泡换气功能
