[单选题]
A He-Ne laser (632.8 nm) is used to calibrate a gratrj. i0 jr oce;ti/8c7ao(m*v h13rqhning. If the first-order h3 2xg1co9xv tmaximum occurs at 32cxx 1 v9ghto$20.5^{\circ}$, what is the grating constant (the distance between the slits)?
A. $1.81 \; mm$
B. $2.20 \; mm$
C. $3.62 \; mm$
D. $4.52 \; mm$
