In the Following APA Formatted Reference, to What Does the 21

In the following APA formatted reference, to what does the 21 refer? Harris, R. M., Bausell, R. B., Scott, D. E., Hetherington, S. E., & Kavanagh, K. H. (1998) . An intervention for changing high-risk HIV behaviors of African American drug-dependent women. Research in Nursing and Health, 21(3) , 239-250.

A) Bibliographical reference
B) Chapter number
C) Issue number
D) Volume number