Passage Summary: Azaleas love acidic dirt. While clay isn't usually acidic, you can mix it with other things to make it that way. Therefore, you can make azaleas grow well in clay.

Conclusion: Azalea bushes can be successfully grown in clay soil.

Reasoning: Azaleas thrive in acidic soil, and clay soil can be modified by mixing it with other soil types to achieve acidity.

Analysis: This argument relies on a missing link between the modification of the soil and the health of the plant. The author assumes that the act of mixing clay with other soils doesn't create a new problem—like poor drainage or toxic mineral levels—that would kill the azaleas despite the correct acidity. To find the necessary assumption, look for an answer that ensures the 'acidifying' process doesn't inadvertently make the soil unsuitable for azaleas in some other way.