Let u with rightwards arrow on top equals open angle brackets fraction numerator square root of 2 over denominator 2 end fraction comma fraction numerator square root of 2 over denominator 2 end fraction close angle brackets. Which of the following is true? D subscript u with rightwards arrow on top end subscript f (4 comma 4 )greater than 0 D subscript u with rightwards arrow on top end subscript f (4 comma 4 )less than 0 D subscript u with rightwards arrow on top end subscript f (4 comma 4 )equals 0