Question

65 million years ago there was a mass extinction that is best known for killing the dinosaurs. there was also a large deposit of dust that contained substances only found in asteroids. what most likely killed off the dinosaurs?
an asteroid impact caused the sun to be blocked.
they were crushed by an asteroid.
the water supplies were polluted.
mammals evolved to be better competitors.

Explanation:

The presence of asteroid - specific substances in the dust deposit suggests an asteroid impact. An asteroid impact could have thrown up enough dust to block the Sun, causing a drop in temperature and disrupting the food chain, leading to the extinction of dinosaurs.

Answer:

An asteroid impact caused the Sun to be blocked.