[单选题]
A He-Ne laser (632.8 nm) is usediw(h5 , dh(xvb6bg kucp9 4,jx to calibrat f0ixewzk; jlphaa0/ l;iu;: 6e a grating. If the first-order maximum occurs:ha f pl0i we;/l;izajx6ku; 0 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. $4.52 \; mm$