What purpose do colons serve in a sentence or phrase?

Why does arthur morgan want to leave the town of blackwater and why did no one else escape except jhon and abigal

The greatest gift the sumerians gave the world was the invention of writing. the sumerians were wealthy people. they needed some way to keep track of what they owned. they began drawing pictures. they used a reed as a pen. they drew on soft pieces of clay. the soft clay was then dried in the sun. the tablet became a permanent record. later, the sumerian drawings changed into wedge-shaped symbols. this kind of writing is called cuneiform. by putting symbols together, the sumerians could write entire sentences.

If the four main points of this informational speech were changed 1. part-time job interviews ii. summer job interviews iii. college entrance interviews iv. full-time job interviews organization type would change from a. topical, cause-effect b. chronological, topical c. spatial, chronological d. cause-effect, spatial