Questions 12 - 15 refer to the following situation. A forest fire destroys trees in a large area. The forest rangers use the burned area to study the effect of different
types of grasses on soil erosion. They choose ten plots of ground that are the
same size. Each of these plots receives the same amount of sun. Each of these
plots also has the same kind of soil and the same slope. The rangers plant each
with a different type of grass. Measurements of soil erosion are made every
week for the entire summer. What is the independent or manipulated variable in
this study?
the types of grasses
the size of the plots
the amount of soil erosion
the type of soil in the plots