Please help :( Which sentence from the passage is complex?

The panther paused to sniff the humid night air.
It heard noises up ahead, but it did not recognize the sounds.
The strange noises continued, and the panther became frightened.
The panther crept into a cave because it was afraid.

"The strange noises continued, and the panther became frightened."

Explanation:

A complex sentence is a sentence that contains an independent clause and one or more dependent clauses. An independent clause can stand alone as a sentence, but a dependent clause even though it has a subject and a verb cannot stand alone.

one is a red flower, and one is the sky when the sun sets.