Answer: C
Explanation: Answer C makes the most sense because "They" is a verb which is used to refer to a group of people, animals, or things. This makes the most sense in the sentence rather than "It" which is used to refer to a singular thing or animal, not a group of people. "Them" is used to refer to a group of people or things that are not the speakers. and Us" is used to refer to the speaker and others, but it does not fit the context of the sentence as it does not indicate that the group will be going to the circus tomorrow.