3points
what is the purpose of writing a topic sentence?

The purpose is to inform the reader on what the story/document/any writing material is going to be about; aka the topic.

read the text, then mark the beginning, the node and the outcome, and point out the characteristics of the quantum manifested in the story

the black sheep

in a distant country a black sheep existed many years ago, was shot.

a century later, the repentant flock raised an equestrian statue that was very good in the park.

thus, in the future, every time that black sheep appeared they were quickly passed by the arms so that the future generations of common sheep could also be exercised in the sculpture