Why did the dinosaur become extinct?

 There are different opinions about the extinction of dinosaurs. Many animals survived in that critical period, including crocodiles, lizards, snakes, tortoises, birds and mammals. If there were some elements caused the extinction of dinosaur, why these animals got survival?

It’s worth considering that dinosaurs lived on land, but many sea animals and terrestrial plants vanished at the same time. These species may disappeared for a seam reason.

Unexpected disaster caused the extinction of most species according to latest opinions. People assume that a aerolite struck the earth 65 million years ago, it caused a great wave and the rocks began to break. After that, the sky was dark for a few months. So plants relying on sunshine died firstly, and then the herbivore vanished. Afterwards, the predators died. The world was cold without sunshine, and then became hot with thinner atmosphere. Some dinosaur survived at that time but eventually died.

If it really happened, we may understand the extinction of dinosaurs. But why did the sensitive animal such as birds get survival? Maybe dinosaur did not die in a sudden moment, but in a gradual time.