Which type of study is this?

a. A prospective observational study because it followed the maids for four weeks.

b. A retrospective observational study because at the end of four weeks, the researcher had to look back at the original measurements.

c. A survey because the maids were randomly selected.

d. A designed experiment because a randomly selected group was treated differently and the researcher was interested in the effect of that treatment.

e. A voluntary response study because the maids volunteered to participate.