The Babylonians were the first to set the measurement of a full circle at 360 degrees. They reasoned that the most important circle is the revolution of the earth around the sun, a period of time which we call a year. They knew that a year was approximately 365 days. However, the number 360 is much more convenient for mathematical computation, since it is divisible by many factors among them, 2, 3, 4, 5, 6, 8, 9, 10, and 12.
What is the topic sentence in the paragraph above?

They knew that a year was approximately 365 days.

They reasoned that the most important circle is the revolution of the earth around the sun, a period of time which we call a year.

The Babylonians were the first to set the measurement of a full circle at 360 degrees.

However, the number 360 is much more convenient for mathematical computation, since it is divisible by many factors among them, 2, 3, 4, 5, 6, 8, 9, 10, and 12.