Passage Summary: Researchers saw that people's brain stress centers lit up on a scan when they lied about a card game, so they think these scans could work as lie detectors.

Conclusion: fMRI technology has the potential to become an effective tool for lie detection.

Reasoning: In a study, subjects who lied about their playing cards showed increased activity in brain regions associated with stress during fMRI scans.

Analysis: The argument assumes that the stress detected by the fMRI is a direct and exclusive result of the act of lying. To weaken this, we should look for an answer that suggests the stress might have been caused by something else in the experimental setup, or that lying doesn't always cause stress in real-world situations. For instance, if the subjects were stressed simply by the act of being in an fMRI machine or by the pressure of the task itself, the link between 'lying' and 'brain activity' is severed. Look for an answer that provides an alternative explanation for the brain activity or suggests the study's results aren't generalizable.