Marina Exam 2017: In a river subject to tidal currents, the best time to dock a ship without the assistance of tugs is _________

In a river subject to tidal currents, the best time to dock a ship without the assistance of tugs is __________. 

A. at slack water 
B. when there is a following current 
C. at high water 
D. at flood tide 


Answer:  
A.at slack water