Solve the Problem $\frac { 1 } { 9 } h r$

Solve the problem.
-Martha can rake the leaves in her yard in 2 hours. Her brother can do the job in 7 hours. How long will it take them to do the job working together?

A) $\frac { 1 } { 9 } h r$
B) $\frac { 14 } { 5 } \mathrm { hr }$
C) $\frac { 1 } { 14 } \mathrm { hr }$
D) $\frac { 14 } { 9 } \mathrm { hr }$