Identify the following.
after the party, the girls sat down to discuss the entertainment and the guests.
fragment
simple sentence
complex sentence

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.

line a --> main

line b --> young

line c --> atitude