Which port city was significant during the Sangam Age for trade?

Read the following excerpt and answer the question that follows:

"The didactic literature, which includes the world famous Tirukkural is mostly in stanzaic form, the stanza having from two to five lines."

(Source: https://www.du.ac.in/up/oads/new-web/04012023_Appendix-108.pdf)

Who authored Tirukkural?

What were herostones, and why were they erected?

What materials were used to inscribe records during the Sangam Age?