Suppose that 20% of students at high school A and 18% of students at high school B participate on a school athletic team. Independent random samples of 30 students from each school are selected and asked if they participate on a school athletic team. Let ˆ p A represent the sample proportion of students at school A who participate on a school athletic team and ˆ p B represent the sample proportion of students at school B who participate on a school athletic team. What is the mean of the sampling distribution of ˆ p A − ˆ p B ?