家兔的配种制度有（）、（）、（）。
参考答案：重复配种；双重配种；频密繁殖
If two sailing vessels are running free with the wind on the same side, which one must keep clear of the other?（）.
A.The one with the wind closest abeam
B.The one to windward
C.The one to leeward
D.The one that sounds the first whistle signal
起重机零件润滑的作用是（）
A、减少磨损
B、防止锈蚀
C、密封防尘
D、提高质量
