Two organisms within the same species can often still have a variety of traits. For example, some humans have straight hair while others may have curly hair, and some even bald. What causes these differences between organisms?

I'm pretty sure it's c. extremely gradual but not 100% sure though.

Humans have survived and thrived in mexico city by using technology to create greater access to land. when the spanish conquered the aztec capital, tenochtitlan, in the 16th century, they built mexico city by draining the lakes and filling in the lake water to create more arable land for agriculture. this technique of land reclamation has continued to be fundamental to human civilization in mexico city. 

Ascenario that doesnt  include any modern objects like electricity

Light rain because storms had lightning and it started crops on fire or destroyed stuff