Question 38$A$ takes $m$ times as long to do a piece of work as $B$ and $C$ together; $B$ takes $n$ times as long as $C$ and $A$ together; and $C$ takes $x$ times as long as $A$ and $B$ together. Then $x,$ in terms of $m$ and $n,$ is:$\dfrac{2mn}{m+n}$$\dfrac1{2(m+n)}$$\dfrac1{m+n-mn}$$\dfrac{1-mn}{m+n+2mn}$$\dfrac{m+n+2}{mn-1}$