In a poll conducted by phone,

I. Some people refused to answer questions.

II. People without telephones could not be in the sample.

III. Some people never answered the phone in several calls. Which of these possible sources of bias is included in the ±2% margin of error announced for the poll?

a. I only

b. II only

c. III only

d. I, II, and III

e. None of these

Select the best answer.