Decide What Values of the Variable Cannot Possibly Be Solutions $\frac { 1 } { 10 } \mathrm { hr }$

Multiple Choice

Decide what values of the variable cannot possibly be solutions for the equation.
-Frank can type a report in 3 hours. James takes 7 hours to type it. How long will it take the two of them typing together?

A) $\frac { 1 } { 10 } \mathrm { hr }$
B) $\frac { 1 } { 21 } \mathrm { hr }$
C) $\frac { 21 } { 10 } \mathrm { hr }$
D) $\frac { 21 } { 4 } \mathrm { hr }$