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What are the conditions for a critical point of a function ? Choose the correct answer below. O A. The point (a.b) is an interior point of the domain of f. Either t, (ab) =0, or ,(a.b) =0. O B. Only that f, (ab)=[(ab). O C. The point (aub) is an interior point of the domain of f. Either f'(a) =0, or I does not exist at a. O D. The point (a.b) is an interior point of the domain of f. Either f,(a.b)=f,(ab)=0, or one (or both) off, or fy does not exist at (a b)