Help, please the question and thank you

quite an interesting way to teach topic sentences, because the answer to this question is the best definition of a topic sentence that one can present, it is as if answering the questions taught you how to do it, this is some real teaching, it is terribly well-done.

the topic sentence in this paragraph is:

topic sentences writers stay on track and keep a paragraph from becoming an aimless collection of sentences about an idea.

which incidentally tells you what a topic sentence should be.

when alex falls for the charming new boy at school, cole -- a handsome, funny, sports star who adores her - she can't believe she's finally found her soul who truly loves and understands her.  

at first, alex is blissfully happy. sure, cole seems a little jealous of her relationship with her close friend zack, but what guy would want his girlfriend spending all her time with another boy? as the months pass, though, alex can no longer ignore cole's small put-downs, pinches, or increasingly violent threats.  

as alex struggles to come to terms with the sweet boyfriend she fell in love with and the boyfriend whose "love" she no longer recognizes, she is forced to choose -- between her "true love" and herself.

explanation: