GMAT Critical Reasoning (CR) | Climatologists believe they know why Earth has undergone a

I hope below explanation would help people having doubts over why D is the correct answer:

Para concludes that CosmicDust causes earth temperature to drop.
But D says Large CosmicRock (maybe large asteroids which killed dinosaur's) impact caused earth dust to cover earth resulting in Earth temperature drop.
So it actuallyweakens the conclusion because it says earth dustnot cosmic dust caused it.
We neglected this because cosmic dust and cosmic rock sounds similar.
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