Jill Is $12 \mathrm{~km}$ Away from Joe $1.5 \mathrm{~km}$ Per Hour If They Meet in 3 Hours, How Fast Is

Jill is $12 \mathrm{~km}$ away from Joe. Both begin to walk toward each other at the same time. Jill walks at $1.5 \mathrm{~km}$ per hour. If they meet in 3 hours, how fast is Joe walking?

A) $1.5 \mathrm{~km}$ per hour
B) $3.5 \mathrm{~km}$ per hour
C) $4.5 \mathrm{~km}$ per hour
D) $2.5 \mathrm{~km}$ per hour