1. Let $a$ be a prime number, and $7 a^{2}+8$ and $8 a^{2}+7$ are also prime numbers. Let $x=77 a+8, y=88 a+7$. Then, among the following scenarios, the one that must be true is ( ).
(A) $x, y$ are both prime numbers
(B) $x, y$ are both composite numbers
(C) One of $x, y$ is a prime number, and the other is a composite number
(D) For different $a$, any of the above may occur
- 1. A.

When $a=3$,
$$
7 a^{2}+8=71 \text { and } 8 a^{2}+7=79
$$

are both prime numbers, and
$$
x=77 a+8=239, y=88 a+7=271
$$

are both prime numbers;
When the prime number $a$ is different from 3, then $a^{2}$ leaves a remainder of 1 when divided by 3.
Let $a^{2}=3 n+1$. Then
$$
\begin{array}{l}
7 a^{2}+8=21 n+15, \\
8 a^{2}+7=24 n+15 .
\end{array}
$$

Neither of them are prime numbers, which contradicts the condition.