A person standing on the bank of a river observes that the angle of elevation of the top of a tree on the opposite bank of the river is $$60^\circ$$ and when he retires $$40$$ meter away from the tree the angle of elevation becomes $$30^\circ$$. The breadth of the river is

A

$$20$$ m

B

$$30$$ m

C

$$40$$ m

D

$$60$$ m