$125 a_{1}=3, a_{2}=7$. When $n \geqslant 1$, $a_{n+2}$ is equal to the unit digit of $a_{n} a_{n+1}$. The 1994th term of the sequence $\left\{a_{n}\right\}$ is
A. 9
B. 7
C. 3
D. 1
125 B. The sequence is $3,7,1,7,7,9,3,7, \cdots$. Starting from page 72, it repeats cyclically, $a_{n+6}=a_{n}$. Therefore, $a_{1994}=a_{2}=7$.