$30 \cdot 16$ If $n=20 \cdot 30 \cdot 40 \cdot 50 \cdot 60 \cdot 70 \cdot 80 \cdot 90 \cdot 100 \cdot 110 \cdot 120$ - 130, then the smallest prime number that is not a factor of $n$ is
(A) 19.
(B) 17.
(C) 13.
(D) None of the above.
(China Shanghai Junior High School Mathematics Competition, 1987)
[Solution] Since $n=2 \cdot 3 \cdot 4 \cdot 5 \cdot \cdots \cdot 9 \cdot 10 \cdot 11 \cdot 12 \cdot 13 \cdot 10^{12}$ $=2^{22} \cdot 3^{5} \cdot 5^{14} \cdot 7 \cdot 11 \cdot 13$,

it is clear that the smallest prime number that is not a factor of $n$ is 17. Therefore, the answer is $(B)$.