防治棉蚜的关键时期在棉花生长的（）、（）期。
参考答案：苗期、蕾期
The train ________ they were travelling was late.[ ]
A. by which
B. to which
C. in which
D. at which
不少食草兽类与其食性和取食行为相适应，消化系统出现了复胃，牛羊具有4室，哪部分属于真正的胃（）。
A.瘤胃
B.网胃
C.瓣胃
D.皱胃
