Step 1: Look at what the question asks and match it to a sentence in the passage.
Step 2: The passage says weariness in man is caused by walking a long distance and exercising the limbs.
Step 3: Compare this with each of the four choices. Only "Walking a long distance" agrees with the passage. The other three are either not in the passage or belong to a different idea.
Step 4: So the correct choice is option 3.
Final Answer: \[\boxed{3}\] (Walking a long distance)
