1. $\mathrm{A}$

The sequence is increasing, each term being the product of two prime numbers, so $a_{18}=3 \times 17=51$.
1. The first 14 terms of the sequence $\left\{a_{n}\right\}$ are $4,6,9,10,14,15,21,22,25,26,33,34,35$, $38, \cdots \cdots$ According to this pattern, $a_{18}$ is ( ).
A. 51
B. 55
C. 57
D. 49