The National AIDS Behavioral Surveys asked a random sample of 2673 adult heterosexuals how many sexual partners they had in the past year. Why is an estimate based on these findings likely to be biased? Select all that apply.

a. Because people might be ashamed to admit that they change partners very often, as this behavior may be considered immoral by some.
b. Because people might fear to admit that they cheat on their spouses.
c. Because only 2673 adults participated in this survey
d. Because the proportion of people having multiple partners must be very small, so an estimate of this would not meet the conditions for using the Central Limit Theorem.