[单选题]
X-rays of wavelength 0.2uiyn8 kinu9 ())znr m+0 nm are scattered by a me-yhq/ con 1sf:tal. The wavelength shift is observed to bn/ -1yhqsc :foe $2.1 \times 10^{-12} \; m$ 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. $80^{\circ}$
