let X be a random variable with density function given by 1 - x si -1gas1 f (z) = 0 en otro caso then 4x (1), donde 4x where , denoted by the characteristic function of X is equal to O a. 2 - 2 cos (1) O b. 1 - cos (2) Oc. 1 O d. 2+ 2 cos (1) O e. 1 + cos (2)