A Researcher Is Studying the Incidence of Child Sexual Abuse

A researcher is studying the incidence of child sexual abuse in public schools and she sends a questionnaire to 1,000 randomly selected students' homes in the month of January.The questions ask how frequently parents engage in certain behaviours like hitting,criticizing,and punishing their children.The same questionnaires are sent to the same participants in June of the same year.However,this time less than 500 questionnaires are returned.This survey faces the problem of:

A) refusal or nonresponse.
B) lack of probability sampling.
C) lack of test-retest reliability.
D) no volunteer bias and accurate memory.