A flag of height 4 metres is standing on the top of a building. The angle of elevation of the top of the flag from a point X is 45° and the angle of elevation of the top of building from X is 30°. Point X is on the ground level. What is the height (in metres) of the building?

A

$$\surd3+2$$

B

$$2(\surd3+1)$$

C

$$4(\surd3+1)$$

D

$$(\surd3+1)$$