The correct answer is - climate change.
The gigantic meteorite that stuck the Earth caused a lot of damage directly, and the impact itself killed a lot of dinosaurs, but what led to the extinctions of them was the drastic change in the climate and in general of the lower atmosphere.
Because of the impact, as well as all the volcanic activity that was triggered of it, enormous amounts of dust and ashes were released into the atmosphere. The whole planet was covered by this dust and ashes, which led to prolonged period of very cold weather, toxic atmosphere (except for the part of it just over half a meter of the ground), and acid rains. Because of the short period in which all this happened, the dinosaurs where not able to adapt and they died off.
