Answer:
All residents of the neighborhood
Step-by-step explanation:
In this case, we are told the residents want to determine the number of dandelions in each yard. But, they decided to treat just some selected yards with a weed-killing substance.
Now the residents want to determine the effectiveness of the weed killer, but, the question they want to answer is "how many dandellions are in your yard?" . Since the question isn't directed to a particular set of people, also the question to be answered isn't "how many dandelions did the weed killer kill?" or "how effective is the weed killer?" The best group to take a sample would be all residents of the neighborhood because they all have dandelions in their yard.
