The answer for me is the letter b and d the b to criticize our own mistakes, it is feasible since the moment you investigate more deeply a subject at the time you review your work again you realize that you were wrong and you can correct it, and the letter d identify and eliminate habitual mistakes is as I commented in letter c when you investigate and delve into a topic you realize your mistakes and normally having a wrong idea of a topic you repeat that error multiple times and the best way to correct it is to review and minimize it.
