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 .
What is the reason for girls being at Kristen 's parents ' house ?
Pick the best answer from the following options:
- It is Kristen 's birthday .
 - It is Kristen 's parents ' birthday .
 - None of the above choices .
 - Kristen invited them over for winter .


It is Kristen 's birthday .