百思不得其解

l7-1-15

16. Comets do not give off their own light but reflect light from other sources, such as the Sun. Scientists estimate the mass of comets by their brightness: the greater a comet’s mass, the more light that comet will reflect. A satellite probe, however, has revealed that the material of which Halley’s comet is composed reflects 60 times less light per unit of mass than had been previously thought.

The statements above, if true, give the most support to which one of the following?

(A) Some comets are composed of material that reflects 60 times more light per unit of mass than the material of which Halley’s comet is composed.
(B) Previous estimates of the mass of Halley’s comet which were base on its brightness were too low.

为什么是b呢，我认为b答案正好相反，请nn指教