Which of the following functions represents a probability density function (pdf)? Select only one answer.
A. f(x) = -2x, for -1 ≤ x ≤ 1
B. f(x) = 1/(x+1), for 0 ≤ x ≤ 2
C. f(x) = 2/(1+x²), for -[infinity] < x < [infinity]
D. f(x) = e(-x), for x ≥ 0