问题
单项选择题
In a river subject to tidal currents，the best time to dock a ship without the assistance of tugs is（）.
A.at high water
B.when there is a following current
C.at slack water
D.at flood
答案
参考答案：C
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在点
(a＞0，b＞0)处沿曲线
1在这点内法线方向的方向导数为______.