Sources are called tertiary sources if they
a. include excerpts from firsthand accounts but are not entirely firsthand accounts.
b. are a collection of raw data instead of being one single piece of raw data.
c. provide information that leads the researcher to primary or secondary sources.
d. were produced by someone close to the subject who didn't actually know the subject personally.

C. provide information that leads the researcher to primary or secondary sources

Sources:
1. Primary
2. Secondary
3. Tertiary

A primary source is a type of obtaining or sourcing information after the phenomenon occurred at that particular time and when photographs are the first-hand materials that were able to capture that specific moment which then can convey the information. Primary resources also include –newspapers, journal articles, magazines and etc. These information are very vital for its credibility and reliability.