A can complete a work in ‘m’ days and B can complete it in ‘n’ days. How many days will it take to complete the work if both A and B work together ?
[A] $(m+n)\ days$
[B] $\left ( \frac{1}{m}\times \frac{1}{n} \right )\ days$
[C] $\left ( \frac{m+n}{mn} \right )\ days$
[D] $\left ( \frac{mn}{m+n} \right )\ days$