Which best describes the purpose of a control sample?

The force acting on a beam was measured under the same operating conditions and a sample of 5 data points was collected. this process was repeated by 3 observers. the pooled standard deviations of these 3 data sets was 1.21. determine the true mean force using all pooled data (with a 95% probability). the result should look like this: true mean = mean +/- margin of error

Place me with this question is my last question

Alandscaper is shopping for landscaping materials. she wants to use materials through which water flows easily. which materials should she choose? check all that apply. clay gravel granite rocks with cracks loosely packed soil