5. If integers $a, b$ cannot be divided by 5, but $a+b$ can be divided by 5, then which of the following expressions cannot be divided by 5? 
(A) $2a+3b$
(B) $2a-3b$
(C) $3a+8b$
(D) $3a-7b$
5. A.

Verify one by one
$$
\begin{array}{l}
2 a-3 b=2(a+b)-5 b, \\
3 a+8 b=3(a+b)+5 b, \\
3 a-7 b=3(a+b)-10 b,
\end{array}
$$

can all be divided by 5.
But $2 a+3 b=2(a+b)+b$, cannot be divided by 5.