Let f (x, y, z) = a + yz and A = (1, 1, -1). Which of the following must be correct? P. The rate of change of f at A in the direction is V2. Q. f does not change at A in the direction . R. There is a unit vector u so that Do f ( A) = 4. O P only O Q and R O P and Q O R only O Q only