A person standing on the bank of river observes that the angle of elevation of the top of a tree on the opposite bank of the river is ${60}^0$ and when he retires 40 meters away from the tree the angle of elevation becomes ${30}^0$. Then the breadth of the river is 

• A

  20 m

• B

  30 m

• C

  40 m

• D

   60 m