The Sun Has Apparent Brightness $B$ At the Earth, Which Is $1.5 \times 10^{8} \mathrm {~km}$

Multiple Choice

The sun has apparent brightness $B$ at the earth, which is $1.5 \times 10^{8} \mathrm {~km}$ away. What would be the apparent brightness of the sun at Pluto, which is $6.0 \times 10 ^ { 9 } \mathrm {~km}$ from the sun?

A) $0.025 \mathrm {~B}$
B) $0.00063 B$
C) $0.25 B$
D) $0.063 \mathrm {~B}$