The time X (in minutes) for a customer to be served at a post office has density function f (x) = 2e−2x, x ≥ 0.

The probability that each of the next three customers will be served within one minute equals

(a) e−6

(b) 3e−2

(c) 2e−6

(d) 2e−3

(e) (1 − e−2)3