Your answer is going to be A.
In Ancient times, sickness was thought to be caused by an excess of blood. Therefore, doctors would cut into their patients and let some of the blood out to help cure them. This theory advanced into thinking that the human body needed all of its "humors" to be balanced: blood was one of the humors so if the doctor thought you had an imbalance, they would 'let' some of the blood out.
In my opinion, the correct answer is <span>C. As Jake was reading the email from June yet again, the flight attendant coughed gently to get his attention.
A complex sentence has one independent clause (that can stand on its own), and two or more dependent (subordinate) clauses. In this case, the independent clause is "the flight attendant coughed gently to get his attention". The dependent one starts with a subordinator "as". The other options are compound sentences - made of two or more independent clauses which are connected with coordinators (but, and).</span>
