4. (5 points) S and T have the following joint density function: f(s,t) = 120st(1 s s - t) for s 0, t 0, s+t 1. The marginal density function for S is (by inspection: little or no computation needed) (a) 20s (1 s) for 0 < s < 1. - (b) 12s(1-ts) for 0 s 1 t. (c) 20s 60s for 0 s 1. - (d) 20s 60s2 for 0 < s