Twiigs is an online polling website that allows users to easily create polls and to vote on polls created by other users. A March 2010 poll asked, "Has concern about autism influenced your decision about whether to vaccinate your child?" Of the 182 votes cast, 80% selected "No, I'm not worried about it." Which type of sample was used in this poll?
a. voluntary response sample
b. stratified random sample
c. multistage random sample
d. simple random sample