Which of the following could not be probability density functions for a continuous random variable? Hint: Find the area under the function f(x). a. f(x) = .20 for 0 s x s 2 O This is not a PDF. ook O This is a valid PDF. sk rint rences b. fx) = .40 for 1 s x s 3 O This is a valid PDF. O This is not a PDF. c. f(x) = xfor 1s xs 3