Certain Planes of a Crystal of Halite Have a Spacing

Multiple Choice

Certain planes of a crystal of halite have a spacing of 0.399 nm.The crystal is irradiated by a beam of x-rays.First order constructive interference occurs when the beam makes an angle of 20° with the planes of the crystal surface.What angle does the beam make with the crystal planes for second order constructive?

A) 37°
B) 40°
C) 43°
D) 46°
E) 49°