For a stone thrown from a lower of unknown height, the maximum range for a projection speed of 10 m/s is obtained for a projection angle of \[30{}^\circ .\] The corresponding distance between the foot of the lower and the point of landing of the stone is

• A. \[10\text{ }m\]              
• B. \[~20\text{ }m\]
• C. \[\left( \text{20/}\sqrt{3} \right)\text{ m  }\!\!~\!\!\text{ }\]
• D.        \[\left( 10/\sqrt{3} \right)\text{ m}\]