By this time , the girls had already gotten in the pool and were playing volleyball with the host 's ( Kristen 's ) parents . The only other kids in the pool were a freshman and a 7th grader . They kept screaming at us to get in the pool because one team needed another member ; no one was budging .

Question with options to choose from: What is the reason for girls being at Kristen 's parents ' house ?
Choices:
[I] None of the above choices .;
[II] Kristen invited them over for winter .;
[III] Kristen invited them over for summer .;
[IV] It is Kristen 's parents ' birthday .;
[III]