Which of the following is a probability density function? 1. f(x) = cos(x) for x E (0, 7/2) and 0 elsewhere. 2. f(x) = cos(x) for x E (0, 7) and 0 elsewhere. 3. f(a) = > |cos(x) | for x E (0, 7) and 0 elsewhere. 4. f(x) = - (cos(x) ) for x E (0, 7) and 0 elsewhere. Select one: O a. 2. and 4. O b. 1. and 3. O c. 2., 3. and 4. O d. 2. and 3. O e. 1., 3. and 4