Can someone give me a one sentence example of distance decay? has to have the words "distance decay" in it.

The Hubble space telescope was observed to distance decay in two years.

There was a distance decay between my friend and me when he moved to Yorkshire because both of us were so far apart we started talking to each other less and less, we used to be best friends but now we kinda lost touch.

distance decay - The declining intensity of any activity, process or function with increasing distance from its point of origin.

that is the definition we learned in our AP human class though the definition might vary depending on what you are looking for 

there is another definition of it on the influences of the spread of culture and its dispersion 

The answer is c) protestant clergy

It's most likely choice a, it makes the most sense in my opinion.