[单选题]
X-rays of wavelength 0.20 nm are scattered by a metal. t1 pvox+ vn1iv(6y9mw The wave-5d43 wo fyxgvlength shift is observevyd-3o4xwg5 fd to be $2.1 \times 10^{-12} \; m$ at a certain scattering angle measured relative to the incoming X-ray. What is the scattering angle?
A. $20^{\circ}$
B. $40^{\circ}$
C. $60^{\circ}$
D. $80^{\circ}$
