A federal agency mailed a survey to 35% of the households in Chicago, New York, Los angeles, and Miami and found that 60% of the people surveyed had commute times over one hour. So, they concluded that 60% of all Americans have commute times over one hour.

Which of the following statements is correct?

A. This method of sampling can be considered both biased and unbiased

B. This method of sampling is neither biased nor unbiased

C. This method of sampling is biased

D. This method of sampling is unbiased ​