Answer:
1. dunking
2. cross
Explanation:
1. The first question is past continuous tense which has to do with a continuing action that was going at a particular point or time in the past. Therefore, the act of dunking basket ball was going on hen Steve hurt his ankle. Therefore, the correct option is a verb with -ing which is "dunking".
2. The herd of antelope is an example of collective noun. And collective noun is a phrase or a word that is employed to stand for a group of things, persons, or animal.
The rule for collective noun is that it should be treated as singular. The only exception is when the context in which it is used shows individual in the group.
In the question, the context in which "the herd of antelope" shows individual the group because each of them has to cross individually. Since there are more than one antelope crossing at the same time, "the herd of antelope" is treated as plural an it takes a plural verb "cross".