Question

The Cretaceous-Tertiary extinction was most likely caused by _____. an asteroid hitting the earth volcanism both a and b none of the above

Answer 1

I think the answer is an astroid hitting the earth.
Hope this helps.

Answer 2

The correct answer is option A, an asteroid hitting the earth

Reason -

The extinction event of Cretaceous is considered as a mass extinction process in which near about 3/4th of the species on planet earth became extinct. The researches conducted (found in geologic record) to study the cause of this massive extinction event,  revealed presence of  iridium metal on the Earth crust which is a rare metal and is usually found in the asteroids or comets. Thus,  hitting of planet earth by the asteroids or comets can be considered as a cause of this massive extinction.