2b) This is a designed experiment because the treatment was intentionally conducted on the teeth to determine the effect of stain removal .Teeth act as the experimental unit and amount of stain removed is the response variable .If a researcher intentionally changes the value of an explanatory variable, and then records the value of the response variable for each group, the study is a designed experiment.
3c) This is a Completely Randomized Design .In a completely randomized design , each experimental unit is randomly assigned to a treatment. Amount of stain removed is the response variable .The various levels of the factor are the treatments in a completely randomized design.
4d)The percentage of stain removed by each treatment is the response variable .
5e) Each tooth was randomly assigned to one of four treatments: gum A, gum B, gum C, or saliva .Gum or saliva is the explanatory variable .Since the treatment is to to determine the ability of chewing gum to remove stains on teeth ,it is qualitative .
6f)The 64 bovine incisors (teeth) is the experimental units.
7g)Each tooth group was placed into a device that simulated a human chewing gum.The process was repeated six times (for a total of 120 minutes of chewing).This could affect the value of the response variable that is fixed at a set level.
8h) The researchers concluded that gums A and B removed significantly more stain than gum C or saliva. In addition, gum C removed significantly more stain than saliva.