Extinction of dinosaurs caused by asteroids

Some say that an asteroid colliding with Earth was responsible for the extinction of the dinosaurs. It is indeed likely that such a collision did occur and that it caused the death of the vast majority of dinosaurs. However, new evidence suggests that perhaps tens of thousands of dinosaurs lived far enough from the point of collision that they were not directly killed by the impact or by the drastic change in temperature when immense dust clouds created by the impact blocked the sun. Some other force must have been responsible for the death of these remaining dinosaurs.

Which of the following, if true, is an assumption made by the author of the above argument?

a. The water supply used by the dinosaurs who survived the asteroid collision and subsequent change in global temperature had not become contaminated by lethal fallout from the collision.
b. A second asteroid impact or some other cosmic event is likely to have killed off the dinosaurs that survived the asteroid impact and global temperature change.
c. At least a few dinosaurs continued to exist for some years after the asteroid collided with Earth.
d. There is another existing theory that does a better job of explaining the extinction of the dinosaurs than the theory involving an asteroid collision with Earth.
e. The change in temperature caused when dust clouds from the asteroid impact blocked the sun could not have lasted for several years.

OA is A; isn't that way out of scope?

Source- Kaplan quizzes