Residents in a neighborhood determine the approximate number of dandelions in each yard. then, they treat some yards with a weed-killing substance. the residents want to answer the question “about how many dandelions are in your yard? ” to determine the effectiveness of the weed killer. from which group would it be best to take a sample?

all residents of the neighborhood
only residents who had their yards treated with weed killer
only residents who did not have their yards treated with weed killer
only residents who still have dandelions in their yards after treatment with weed killer