种群数量的年间变动，有的是周期性的波动，有的是______的波动。
参考答案：非周期性
It is not such the language_________the cultural background ___________ makes the book
difficult to understand.[ ]
A. but; that
B. nor; which
C. as ; that
D. like ; which
万吨、8～9万吨、20～30万吨船靠栈桥时入泊速度应控制于：（）.
A.＜25cm/s；＜15cm/s；10～15cm/s的范围
B.＜10cm/s；2～8cm/s；1～5cm/s的范围
C.＜5cm/s；＜3cm/s；＜2cm/s的范围
D.＜20cm/s；＜15cm/s；＜10cm/s的范围
