f(x) is defined for x > −1 and has a continuous derivate. If f satisfies f(0) = 1, f′(0) = 0;
(1 + f(x)) f′′(x) = 1 + x. If x is positive then f′(x) is
(A) always positive (B) always negative
(C) always non-negative (D) none of these
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