Which of the following laws states that "good absorbers of heat are good emitters"?

$Assertion :$ $A$ body that is a good radiator is also a good absorber of radiation at a given wavelength.
$Reason :$ According to Kirchhoff's law,the absorptivity of a body is equal to its emissivity at a given wavelength.

For a perfectly black body,its absorptive power is

The filament of a light bulb has a surface area of $64 \ mm^2$. The filament can be considered as a black body at a temperature of $2500 \ K$,emitting radiation like a point source when viewed from afar. At night,the light bulb is observed from a distance of $100 \ m$. Assume the pupil of the observer's eye to be circular with a radius of $3 \ mm$. Then:
(Take Stefan-Boltzmann constant $= 5.67 \times 10^{-8} \ W \ m^{-2} \ K^{-4}$,Wien's displacement constant $= 2.90 \times 10^{-3} \ m \ K$,Planck's constant $= 6.63 \times 10^{-34} \ J \ s$,speed of light in vacuum $= 3.00 \times 10^8 \ m \ s^{-1}$)
$(A)$ The power radiated by the filament is in the range $642 \ W$ to $645 \ W$.
$(B)$ The radiated power entering into one eye of the observer is in the range $3.15 \times 10^{-8} \ W$ to $3.25 \times 10^{-8} \ W$.
$(C)$ The wavelength corresponding to the maximum intensity of light is $1160 \ nm$.
$(D)$ Taking the average wavelength of emitted radiation to be $1740 \ nm$,the total number of photons entering per second into one eye of the observer is in the range $2.75 \times 10^{11}$ to $2.85 \times 10^{11}$.

The intensity of direct sunlight on a surface normal to the rays is $I_0$. What is the intensity of direct sunlight on a surface,whose normal makes an angle of $60^{\circ}$ with the rays of the sun?

The absorption coefficient value of a perfect black body is