X-Ray Diffraction (XRD) Makes Use of the Bragg Equation in Which

X-ray diffraction (XRD) makes use of the Bragg equation in which $\theta$ is the __________

A) wavelength of the X-ray.
B) the distance between layers of identical particles in the lattice.
C) the angle of incidence of the X-ray beam.
D) the dimension of the unit cell.
E) angle of diffraction of the X-ray beam.