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. $a=3 n+1, b=5 n+2$
$(3 n+1,5 n+2)=(3 n+1,3(5 n+2)-5(3 n+1))=1$, so the answer is $D$.
