When you use math in the real world, you don't have multiple choice answers: usually you are solving a problem that nobody has done before. So when you are learning, or doing exams, it is always better to try to let the math solve the problem, and use the multiple choice answers to check your calculation; not to use the answers as plug-ins, to try to solve the problem.
There are real world problems that you have to work backwards, like examining the trajectory of a particle and trying to figure out what forces made it take that path. Those aren't the problems that you get in exams!
Another reason to be wary of multiple choice answers is that they give the examiner the opportunity to tempt you with answers that sound right, but aren't! Much safer to let the math do the work!
Sometimes you may see an exam problem that can only be solved by plugging answers. If so, shame on the examiner, and use it as a last resort.
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