$($numbers $15-17)$ A $4$ kg block with a $1$ kg block stacked on top initially slides at $2$ m$/$s on a rough, horizontal

surface. The blocks continue to move as a unit with no sliding between the two. The blocks come to rest $1$ m

away.

$15.$ How much work did kinetic friction from the surface below do on the $4$ kg block during this process?

a$)-2$ J

b$)-6$ J

c$)-10$ J

d$)-14$ J

e$)-18$ J