Answer:
This is called cluster sampling procedure.
Explanation:
In this type of procedure, a group of elements is considered instead of individual units. This proceddure is usually based on geography, and it's useful due to its efficiency regarding cost and time.
There are a few disadvantages, though: it might not be diverse enough to be significant, and it's possible that some elements of other groups (in this case, other districts) share some features.
The answer is to keep out invaders<span>
</span>
True I believe. Children often tattle!
