Sexual research is particularly prone to volunteer bias because ____. Therefore, only a select group ...

Sexual research is particularly prone to volunteer bias because ____. Therefore, only a select group of people may be willing to participate.
 
  a. there are usually great physical risks involved
  b. it offers less monetary compensation to research participants than other areas of psychological research
  c. sexual research involves topics typically viewed as very private and are likely to evoke strong emotions and embarrassment from many people
  d. participating in this kind of research will show up on one's permanent record affecting future job prospects