Comets do not give off their own light but reflect light from other sources, such as the Sun.
Facts
causal / anti-causal
comets don't give off light, comes do reflect light from other sources (like the Sun)
rule (volume dial)
The more mass, the more light it reflects.
pivot (however)
It turns out that the material in Halley's comet reflects way less light per unit of mass than we used think.
Evaluate
When the final fact in an Inference stimulus has a Pivot, that fact is usually creating some tension with what came before it, and the correct answer usually addresses that tension.
The overlap between the last two claims is "light reflected".
We find out that we were wrong about Halley's comet's light reflecting properties.
Since we know that scientists estimate mass based on reflected light, that would seem to imply that we were also wrong about Halley's mass.
Goal
Let's look for something saying that we need to revise our assessment of Halley's comet mass, since we base that on brightness and we were confused about what material we were measuring.
Since Halley's comet turns out to be less reflective than normal, we probably thought it was 60 times less massive than it really is.