[单选题]
A He-Ne laser (632.8 nm) is used to calibrate a grating. If the first-order )dn6w68 sncp b7m/+ko)pqtf7 ros/czd dp *x6maximum occurs aq*7 6ksodz / cfdo/mtrp)7p+ xt $20.5^{\circ}$, what is the grating constant (the distance between the slits)?
A. $1.81 \; mm$
B. $2.20 \; mm$
C. $3.62 \; mm$
D. $4.52 \; mm$
