To determine whether voters would be willing to vote for a tax increase to improve the city's schools, the board of education surveys 500 parents of children in the city's schools.

why is this sample likely to be biased?

a. the entire population of the city was not surveyed.

b. parents of school children are too busy to be bothered with surveys.

c. there are not enough people in the sample.

d. parents of school children are more likely to be willing to pay more to improve schools.