An environmental agency took a map of a region near a former industrial waste dump and placed a grid of 538 squares on it. They divided the plots into 18 groups and collected soil samples from every plot in three of those groups and checked each for evidence of toxic chemicals. Complete parts a) through c) below. a) What type of sampling did they use? Choose the correct answer below. cluster sample census simple random sample systematic sample b) Is there any sort of bias associated with this sampling procedure? Choose the correct answer below. 0 A. Nonresponse bias k 0 B. Response bias 0 C. Undercoverage 6) D. There is no bias associated with this procedure. c) One researcher suggests that plots closer to the old dump site could contain more contaminants than those farther away. How could the sampling procedure be improved to take this into account? 6) A. Use a stratied random sampte. stratifying by distance from the dump site. 0 B. Use a convenience sample, selecting the 18 plots closest to the dump site. 0 C. Use a stratied random sample, stratifying by distance from the nearest road. 0 D. Use a multistage sample. stratifying by distance from the dump site and then using a cluster sample within each