Write PS for Primary Source, SS for Secondary Source and TS for Tertiary Source,
21. Biographies
22. Commentaries
23. Bibliographies
24. Journal Articles
25. Library Catalogs
26. Journals
27. Birth Certificates
28. Guidebooks and manuals
29. Recordings (video and audio)
30. Maps
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Answer:
21. SS
22. SS
23 SS
24. SS
25. TS
26. SS
27. PS
28. SS
29. PS
30. TS
Explanation:
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