Question

[1] When the twins grew up, they continued to fight. [2] One day they decided to hold a contest to determine who was stronger. [3] Though they tried to defeat each other in many different ways, neither one could win. [4] On the last day, they fought with weapons. [5] Using a deer antler, the right-handed twin overpowered his brother, and he threw him off the edge of the Earth. [6] Today, the right-handed twin lives in the Sky-World, and the left-handed twin lives in the dark underworld. [7] They rule the earthly world of men together. Which best describes sentence 7?

Answer 1

[3] Though they tried to defeat each other in many different ways, neither one would win. 

This is the best answer, in my opinion, that describes [7]. Sentence 7 talks about how the twins rule the earthly world of men together, and sentence 3 says that even though they tried to defeat each other, no one won. This basically means that after they tried to defeat each other, they gave up and started ruling together.

Answer 2

It is a simple sentence.

A simple sentence contains one independent clause - which is the case with sentence 7. There is one subject (they) and one verb (rule), which means that it is a simple sentence (because of one verb only). A dependent clause wouldn't be able to stand on its own, and sentence 7 obviously can, which is why it is not dependent.