To determine whether voters would be willing to vote for a tax increase to improve the city's parks, the town mayor surveys 100 parents of children in the city's schools. Why is this sample likely to be biased?
   Parents of school children are too busy to be bothered with surveys.
   There are not enough people in the sample.
   The entire population of the city was not surveyed.
    Parents of school children would be more willing to pay to improve parks.