Which sentence below is a complex sentence?
a. some students like to study in the morning.
c. when he handed in his homework he forgot to give the teacher the last page.
b. i tried to speak spanish and my friend tried to speak english.
d. the cat lived in the backyard, but the dog, who knew he was superior, lived inside the house.

[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.