A study is conducted to determine the level of confidence the public has in the ability of the federal government to respond to a natural disaster. A random sample of 500 people was taken from each of the five major ethnic groups in the USA. A survey was given to each of the 2500 participants and a measure of confidence was computed for each participant. These measures were then summarized into an overall average level of confidence for the public as a whole. This type of study is an example of

A. a double blind study.

B. an observational retrospective study.

C. a stratified random sample.

D. a prospective case-control study.

E. none of the above