Which of the following is a complex sentence? Even though I enjoy music, the song was pretty loud.
As far as the sky, that's how much I love Christmas.
The song was pretty loud after being turned up.
Since August, the days have been getting shorter,

Even though I enjoy music, the song was pretty loud.

Explanation:

Complex sentences have one independent clause and at least one dependent clause

the following key ideas correctly express the key sentence.

restate key idea: most people can learn to swim in ten short lessons.

restate key idea: the majority of water-accident victims require mouth-to-mouth breathing.

restate key idea: an empty, tightly closed gallon jug will support a tired swimmer.  

restate key idea: first aid is to prevent accidents as well as assist in rescue.

explanation: the key idea needs to have the same topic as the key sentence (drowning and safety).