2. Let $n$ be a positive integer, among the following pairs of numbers $a, b$, the one that makes $\frac{b}{a}$ a reduced fraction is ( ).
(A) $a=n+1, b=2 n-1$
(B) $a=2 n-1, b=5 n+2$
(C) $a=n+1, b=3 n+1$
(D) $a=3 n+1, b=5 n+2$
2. D.

Notice,
$$
\begin{array}{l}
(3 n+1,5 n+2)=(3 n+1,2 n+1) \\
=(n, 2 n+1)=(n, n+1)=1 .
\end{array}
$$

Therefore, $\frac{5 n+2}{3 n+1}$ is a reduced fraction.