We have 12 separate plots of soil sampled from a wetland in a controlled laboratory environment. We subject each to simulated snowmelt, with six plots randomly assigned to be treated with neutral pH snowmelt and the other six plots treated with reduced pH snowmelt. The assignment was done according to complete randomization. The outcome of interest is the number of Copepoda (small crustaceans) from each microcosm during the first 14 days of snowmelt. The (fake) data is given in the table below. You can take neutral pH to be the "control" and reduced pH to be the "treatment."

Reduced pH Neutral pH 250 160 150 57 125 250 180 161 147 78 129 181 a) What are the experimental units? b) Estimate the average treatment effect. That is, find +. c) Estimate the variance of the average treatment effect. That is, find var(+)