Decide What Values of the Variable Cannot Possibly Be Solutions $\frac { 24 } { 7 } \mathrm { hr }$

Decide what values of the variable cannot possibly be solutions for the equation.
-Martha can rake the leaves in her yard in 6 hours. Her brother can do the job in 8 hours. How long will it take them to do the job working together?

A) $\frac { 24 } { 7 } \mathrm { hr }$

B) $\frac { 48 } { 2 } \mathrm { hr }$

C) $\frac { 1 } { 14 } \mathrm { hr }$

D) $\frac { 1 } { 48 } \mathrm { hr }$