Aradio talk show host asks listeners to call the radio station and give their opinions on a proposed new city ordinance. he uses the opinions of 200 callers to conclude that the voters are overwhelmingly opposed to the ordinance.

why is this sample likely to be biased?

a.
the survey is voluntary and includes only listeners of this radio program.

b.
there are not enough people in the sample.

c.
radio listeners are more likely to be opposed to city ordinances.

d.
the entire population of the city was not surveyed.