Geneticists hypothesized that one reason for a prolonged lifespan observed in yeast cells treated with the common drug ibuprofen stems from inhibiting the uptake of certain amino acids. In an experiment, they compared levels of amino acids in cells treated with ibuprofen to cells from an untreated control group. For the amino acid tryptophan, the six measurements in the treated group had a mean level of 0.0067 with a standard deviation of 0.0038. The six measurements in the control group had a mean level of 0.0080 with a standard deviation of 0.0050.
a. Identify the response variable and explanatory variable. Indicate whether each is quantitative or categorical.
b. Find the standard error of the sample mean in each group and the standard error of the difference in the sample means.

c. A 95% confidence interval for the difference between the mean level of tryptophan in the treated versus the control group is ( -0.007, 0.004). Interpret.