18. A uniformly mass-distributed cubic die is randomly rolled repeatedly until an odd number appears. Then, the probability that each even number appears at least once before an odd number appears is ( ).
(A) $\frac{1}{120}$
(B) $\frac{1}{32}$
(C) $\frac{1}{20}$
(D) $\frac{3}{20}$
(E) $\frac{1}{6}$
18. C.
$$
P=\frac{3}{6} \times \frac{2}{5} \times \frac{1}{4}=\frac{1}{20} \text {. }
$$