18. letf:r" > r" be a differential function. let df(x) be the derivative of f at xe r". which of the following is/are correct? a (a) df(0)u) = o for all u in r

18. Letf:R" > R" be a differential function. Let Df(x) be the derivative of f at xe R". Which of the following is/are correct? A (a) Df(0)u) = O for all u in R (b) Df(x)u) = 0 for all u in Rm and some xe R" only if f is a Constant (c) Df(x)(u) = Ofor allu e R and all x e R" only if fis a constant (d) If f is not a constant function, then DfX) is a one-to-one the unit discl 2 <1} function for some xE R"