混合对渔业（水交换）影响？
参考答案：
使湖水热量从表层输送到底层可使水体中溶解气体离子悬浮物分布均匀，营养物质从底部提供到表层，供表层生物利用
The helm command PORT TWENTY means（）.
A.change course twenty degrees to the left
B.put the rudder left twenty degrees
C.put the rudder hard left for the first twenty degrees of swing
D.put the rudder left twenty degrees and then ease back as the vessel starts swinging
