ANSWER
They must have common denominators.
EXPLANATION
Given any two rational expressions say,
[tex] \frac{p}{n} \: \: and \: \: \frac{q}{m} [/tex]
where m,n,p,q are integers, and n,m≠0
We can only add these two rational expressions if they have a common denominator.
That is why we find LCD to be mn.
Then
[tex] \frac{p}{n} \: \: + \: \: \frac{q}{m} = \frac{mp + nq}{mn} [/tex]
The last choice is correct.
