What is the next number in the series: 21, 21, 23, 20, 5, 25, 31, 24,?

I came across a puzzle from a workbook for primary school students who wish to sit in exams for enrolling to selective high schools, which asks about what is the next number in the series.

20. What is the next number in the series?

    21, 21, 23, 20, 5, 25, 31, 24, ?

    (A) 3
    (B) 10
    (C) 17
    (D) 86

^{(Original image)}

I've thought many possibilities but none is satisfactory.

What is the next number?