Which of the following statements about record-based perturbation controls is false?

a. They calculate a statistic on the basis of a random sample of records that satisfy the query

b. They calculate a statistic after some type of error term has been added to the data in the records that satisfy the query

c. They can be compromised through an averaging attack if the error term has been generated from a distribution with a zero mean

d. They introduce an error term after the true statistic has been calculated on the basis of the data in the records that satisfy the query