Which of the following contains a linking verb?
a. mateo became upset with me today
b. i took his last cupcake with out permission
c. he wanted to et it this afternoon
d. i bought him a new one as a surprise

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.