Martha Can Rake the Leaves in Her Yard in 4 $7 \mathrm{hr}$

Multiple Choice

Martha can rake the leaves in her yard in 4 hours. Her younger brother can do the job in 7 hours. How long will it take them to do the job if they work together?

A) $7 \mathrm{hr}$
B) $\frac{28}{3} \mathrm{hr}$
C) $\frac{11}{28} \mathrm{hr}$
D) $\frac{28}{11} \mathrm{hr}$