A cone is cut along its slant height and unfolded to from a sector. If the slant height and radius of the cone are respectively l and r, then the area (in sq.units) of the sector is

A

$$\pi lr$$

B

$$\frac{1}{2} lr$$

C

$$\pi l^{2}r^{2}$$

D

$$\frac{1}{2}\pi lr$$