Fill in the blanks:
$(a)$ $1 \, rad = \dots \text{ degrees}$
$(b)$ Light year is a unit of $\dots$
$(c)$ The solid angle subtended by a hemisphere at its center is $\dots$

(N/A) Since $\pi \, rad = 180^{\circ}$,therefore $1 \, rad = \frac{180}{\pi} \approx 57.3^{\circ}$.
$(b)$ $A$ light year is the distance that light travels in a vacuum in one year,so it is a unit of distance.
$(c)$ The solid angle subtended by a sphere at its center is $4\pi \, sr$. Since a hemisphere is half of a sphere,the solid angle subtended by a hemisphere at its center is $\frac{4\pi}{2} = 2\pi \, sr$.