$15 \cdot 21$ The proposition "If $n$ is a positive odd number, and the sum of its digits is 4, and none of them are 0, then $n$ is a prime number." How many numbers can serve as counterexamples to the above proposition?
(A) 0.
(B) 1.
(C) 2.
(D) 3.
(E) 4.
(45th American High School Mathematics Examination, 1994)
[Solution] The odd numbers that meet the proposition conditions are as follows:
$$
13,31,121,211,1111 .
$$

Among them, 121 and 1111 are not prime numbers.
Therefore, the answer is $(C)$.