The square of 1 is 1

The square of 2 is 4.

3~9

4~ 16

So, 5 ~ 25. Analogy.

the second group

A * * * is four words, and the first three groups begin with

I, learn to count, so the fourth should be love.

Then the second word in each group is love, me and study, so the second word in the fourth group should be number, and so on.