Answer:
The best pronoun to complete the sentence and explains its use is:
them; direct object
Step-by-step explanation:
Remember the direct object is the receiver of the action of a verb, while the indirect object is a complementary element that will be accompanying the direct object to provide more information about the object and the action. In other words, they can be a subject but not one modified by the verb. In this case, the only one not modified by a verb is the monster truck race, because it has no verb to be modified.