In a Study of Parents' Perceptions of Their Children's Size

In a study of parents' perceptions of their children's size, researchers asked parents to estimate their youngest child's height.The researchers hypothesize that parents tend to underestimate their youngest child's size because the youngest child is the baby of the family and all other family members appear larger compared to the baby.The 39 parents who were surveyed underestimated their youngest child's height by 7.5 cm, on average; the standard deviation for the difference in actual heights and estimated heights was 0.72 cm, and the data were approximately normal.Which of the following is an appropriate null hypothesis for this study?

A) Parents' estimate of their youngest child's height is close to the real height, on average.
B) Parents tend to overestimate their youngest child's height, on average.
C) Parents tend to underestimate their youngest child's height, on average.
D) None of the above.