[单选题]
A He-Ne laser (632.8 nm) is used tos ztxdx502 )lxvzc-1h i/mz i6 calibrate a g-: 3:uevpkleo-k qf7qrating. If the first-order maximum occurs at 7-eq:ufo p l:kv 3q-ke$20.5^{\circ}$, what is the grating constant (the distance between the slits)?
A. $1.81 \; mm$
B. $2.20 \; mm$
C. $3.62 \; mm$
D. $4.52 \; mm$
