The stream of a river is flowing with a speed of $2\mathrm{ }\mathrm{k}\mathrm{m} {\mathrm{h}}^{-1}$. A swimmer can swim at a speed of $4\mathrm{ }\mathrm{km} {\mathrm{h}}^{-1}$. The direction of the swimmer with respect to the flow of the river, to cross the river straight, is

• A

  $150°$
• B

  $90°$
• Correct answer
  C

  $120°$
• D

  $60°$

Solution

To reach the point directly opposite to the starting point, the resultant velocity of swimmer should be as shown in figure.


 
From figure
$\sin \mathrm{\theta }=\frac{2}{4}=\frac{1}{2}$
$\mathrm{\theta }=30°$
Angle with flow $=90°+30°=120°$

Hence, the correct answer is option: 'C'.