[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] after the right-handed twin chose a deer antler as a weapon, he 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 sentence is a compound-complex sentence?

The correct answer is [5] After the right-handed twin chose a deer antler as a weapon, he overpowered his brother,  and he threw him off the edge of the Earth.

Explanation:

A compound-complex sentence is formed by two independent clauses and a dependent one. It's worth remembering that an independent clause  has a subject and a predicate and is capable of expressing a complete thought on its own. On the other hand, a dependent or subordinate clause is formed by a subject and a verb and starts with a subordinate conjunction or a relative pronoun. A subordinate clause does not express a full idea on its own; it needs the information expressed by the main clause in order to make sense. Dependent clauses commonly begin with words such as after, although, because, before, if, since etc.

Sentence 5 has two independent clauses and one dependent one. The first clause, "After the right-handed twin chose a deer antler as a weapon," is the dependent clause. It begins with after and needs more information to make complete sense. The other two sentences express complete thoughts on their own: "he overpowered his brother,  and he threw him off the edge of the Earth." They are joined by a comma and the coordinate conjunction "and".