[单选题]
X-rays of wavelength 0.2o pq)qv ;rrp057*ya rv0 nj tvw4v* z-+z(m1o iqom are scattered by a metal. The wavelength shift is observezoj4 v+qiw1m(-tvo*z d 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. $80^{\circ}$
