nishita84 wrote:

Simple method, always works - assume (r,t) = (1,1) and (r,t) = (-1,-1)

since the ans from both the cases are disparate, we can conclude that D is the ans.

Yes plugging in numbers is

usually simpler. I always try to display an alternative algebraic solution as another tool in your toolset. You’ll come up with some QC problems where plugging in numbers will actually take longer or actually get you the wrong answer by not putting in all the cases (this especially happens with inequality or absolute value QC problems)

One good thing about algebraic solutions that it applies for all cases, some QC problems are simply just set up for them, and sometimes they are faster approach.

This problem is definitely is not suited for this type of solution for many, but if you work this part of your brain and try to find an algebraic solution for QC problems with variables, it will come in as a handy resource when you’re strapped for time and you can look at the problem algebraically without doing much work and solve the problem.