Passage Summary: Archaeologists believe ancient people moved massive 40-ton stones across a lake using reed boats, and they tested this by successfully moving a much smaller 9-ton stone the same way.

Conclusion: The 40-ton stones at Tiwanaku were likely transported across the lake using reed boats.

Reasoning: Researchers successfully moved a 9-ton stone using a reed boat constructed with traditional local materials and techniques.

Analysis: To evaluate this argument, we need to address the significant weight gap between the experimental 9-ton stone and the actual 40-ton stones. The archaeologists are assuming that success with a smaller load proves the feasibility of a load more than four times heavier. Look for an answer that asks whether the reed boat technology used in the experiment can actually be scaled up to support 40 tons. If the boats have a maximum capacity well below 40 tons, the hypothesis is in serious trouble.