From a point on one bank of a river, the angle of elevation of the top of a tree on the opposite bank is 30∘. If the height of the tree is 12 m, calculate the width of the river.\,(Take tan30∘=31; give your answer to one decimal place.)
a) 13.9 m
b) 20.8 m
c) 6.9 m
d) 24.0 m