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: What is the reason for girls being at Kristen 's parents ' house ?
Answer: It is Kristen 's birthday .It is Kristen 's parents ' birthday .None of the above choices .Kristen invited them over for winter .