Question

Look at the second body paragraph of the skeleton composition. Which part of the topic sentence shown supports the main idea (thesis) of the composition?

Introduction (ending with thesis): Almost all sports, whether indoor or outdoor, require stamina and physical fitness . . . Perhaps the most demanding sport requiring the most muscle stamina, heart strength, and lung action is marathon running.

First Body Paragraph: Although many sports provide very strenuous exercise, they do not utilize all the major muscles in the way that marathon running does . . .

Second Body Paragraph: Although it is a true that a runner may die from a heart attack, running in general actually strengthens the heart . . .

Third Body Paragraph: The most unique benefit marathon running provides is its variety of terrain that strengthens the runner's lungs and therefore stamina better than any other sport . . .

Conclusion (restates thesis): . . . Therefore, running builds muscle stamina, heart strength, and lung capacity better than any other sport.

running in general actually strengthens the heart
that a runner may die from a heart attack
running in general
although it is true

Answer 1

"running in general actually strengthens the heart" — this clause supports the thesis by claiming marathon running builds heart strength.