Which sentences are complex sentences? Check all that apply. You will need to talk to Mr. Cortez if you want to try out for the play. Brooke is a strong athlete, and she is also a hardworking student. Although many people dislike reptiles, I think snakes are fascinating. Elijah wants to buy a guitar, but he hasn't saved up enough money. Once I finish knitting this scarf, I will make a matching hat.

The answers are ...

You will need to talk to Mr. Cortez if you want to try out for the play. Although many people dislike reptiles, I think snakes are fascinating. Once I finish knitting this scarf, I will make a matching hat.

The complex sentences are indeed:

You will need to talk to Mr. Cortez if you want to try out for the play.

Although many people dislike reptiles, I think snakes are fascinating.

Once I finish knitting this scarf, I will make a matching hat.

Explanation:

A complex sentence is formed by joining an independent clause and a dependent one. An independent clause can express a full thought on its own. On the other hand, a dependent clause needs the independent one to give it context, so that it makes sense. It does not express a complete thought on its own. The independent and dependent clauses are joined by subordinating conjunctions. The subordinating conjunction "if" expresses a condition; "although" expresses contrast; and "once" expresses a transition of time. They introduce the dependent clause.

With the explanation above, we can conclude that the correct options are:

You will need to talk to Mr. Cortez if you want to try out for the play.

Although many people dislike reptiles, I think snakes are fascinating.

Once I finish knitting this scarf, I will make a matching hat.