A man in a row boat must get from point A to point B on the opposite bank of the river (see figure). The distance\[BC=\text{ }a\]. The width of the river\[AC=b\]. At what minimum speed u relative to the still water should the boat travel to reach the point B? The velocity of flow of the river is \[{{v}_{0}}\].

• A. \[\sqrt{{{\text{a}}^{2}}\text{+}{{\text{b}}^{2}}}/{{\text{v}}_{0}}\] 
• B.        \[\frac{{{\text{v}}_{0}}\text{b}}{\sqrt{{{\text{a}}^{2}}\text{+}{{\text{b}}^{2}}}}\]
• C. \[{{\text{v}}_{0}}\,\text{a/b}\] 
• D.        \[{{\text{v}}_{0}}\,\text{b/a}\]