A box contains four slips of paper marked 6, 7, 8, and 9. Two slips are selected without replacement. List the possible values for each of the following random variables shown below. (Enter your answers as comma-separated lists.)

(a) x = sum of the two numbers (

b) y = difference between the first and second numbers

(c) z = number of slips selected that show an even number

(d) w = number of slips selected that show a 7