Isaac is researching Abraham Lincoln’s life for a social studies report. He finds an encyclopedia that includes original letters and articles found in historical journals. The type of source he is using is primary. first. secondary. tertiary.

Tertiary

Explanation:

Sources of information could be classed into primary, secondary and tertiary usually depending on the conveyor or narrator of such information. In simple terms, account of an event narrated by those involved, without interpretation or addition editing or additional add ons may be regarded as primary e.g direct interviews, diary information.while an account of an event narrated maybe by a reader or for reporting purposes are classed as secondary such as textbooks, commentaries. Tertiary sources tries to combine both primary and tertiary together by series of account on a particular topic, some of which may have been quoted directly from the subject and some reported by or influenced by discretion. Eaxmaples of tertiary are encyvkpedoan, dictionaries.