In a study of sexuality, 500 possible participants were considered for a sample. These people were contacted and asked to participate in the study. Half of them agreed to participate. However, a majority of the participants were male. Which of the following problems is this study most likely to have due to high male participation?
A. Observer bias
B. Probability sampling
C. Volunteer bias
D. Concealment

C. Volunteer bias        

Explanation:

Volunteer bias occurs in a study when the number of people who willing participate in a study is skewed towards a particular sect, gender, race or class. That is the participants, respondents or subjects of the experiment are not a representation of the general population of interest. For example, in the scenario presented in the question, the volunteers are skewed towards males and the study on sexuality is aimed at the general population of males and females. When this bias occurs, the validity of the result is reduced as the result of the study cannot be applied to the initially intended population, hence the external validity of the result of the study is threatened.  

answer; i believe the first answer to be (independent; /// and the second answer is (french); good luck