[单选题]
A He-Ne laser (632.8 nm) is used to calibrate a grating. If the figpb lnoq (99a 9*t q1;mqwwov5rst-order maxp e)piw/* j:hsimum occur* esph/wjp :)is at $20.5^{\circ}$, what is the grating constant (the distance between the slits)?
A. $1.81 \; mm$
B. $2.20 \; mm$
C. $3.62 \; mm$
D. $4.52 \; mm$
