Question 12 If f (x, y) = yet -4, which of the following is/are correct? P. f has exactly one critical point (2,0). Q. The Extreme Value Theorem guarantees that the maximum value of f on D must occur at boundary point(s) of any closed bounded region D. R. If (a,b) is a critical point of f, then Da f(a, b) = 0 for any unit vector u. O P and Q O P and R O P only O Q only O R only