WHy not C?

[tanyajoseph]

Is rst = 1

1) rs = 1
2) st = 1

[erdnah]

1) not suff
2) not suff
= a, b, d eliminated

r=s=t=1 => rst = 1
r= 1/3, s = 3, t = 1/3 => rst = 1/3

so: e