$30 \cdot 11 \quad 47$ represents the number of ways it can be expressed as the sum of two prime numbers is
(A) 0.
(B) 1.
(C) 2.
(D) 3.
(E) more than 3.
(5th American Junior High School Mathematics Examination, 1989)
[Solution] 47 is an odd number, and all prime numbers except 2 are odd.
Thus, from “odd number + odd number = even number” and “even number + odd number = odd number”, we know:
When 47 is expressed as the sum of two prime numbers, one of them must be 2, making the other 45, which is not a prime number.
Therefore, 47 cannot be expressed as the sum of two prime numbers.
Hence, the answer is $(A)$.