An A grade should be given if you receive the solution from the Experts; you should consider the A grade the default unless it is deficient.
A B grade should be given if the solutions require you to do a good amount of extra work to resolve your problem.
A C grade is the lowest possible grade you can give. Experts should always be given the opportunity to improve it -- but that does not mean they should rewrite your entire application for you.
It is customary to explain any grade that is not an A.
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