A

$\displaystyle\sum\limits_{n=1}^{\infty}\dfrac1n$

B

$\displaystyle\sum\limits_{n=1}^{\infty}(-1)^n\dfrac1n$

C

$\displaystyle\sum\limits_{n=1}^{\infty}\dfrac1{n^2}$

D

$\displaystyle\sum\limits_{n=1}^{\infty}(-1)^n\dfrac1{n^2}$