When p = 12, t = 2, and s = 1/6, r = 18. If r varies directly with p and inversely with the product of s and t, what is the constant of variation?
A: 1/8
B: 1/2
C: 4 1/2
D: 18
A: 1/8
B: 1/2
C: 4 1/2
D: 18