A parallel beam of monochromatic light of wavelength $\lambda$ passes through a slit of width $b$. After passing through the slit the light is incident on a distant screen. The angular width of the central maximum is

A.     ${\text{2}}\frac{\lambda }{b}$ radians.

B.     $\frac{\lambda }{b}$ radians.

C.     $2\frac{\lambda }{b}$ degrees.

D.     $\frac{\lambda }{b}$ degrees.

A