A 'decaying' function is one where 'y' gets smaller as 'x' gets bigger.
You might say that 'y' peters out as time goes on.
Each of the functions listed in this problem have kind of the same form.
For all of them, we can ignore the first fraction, and just look at the fraction
in the parentheses that's raised to the power of +x or -x.
What we're looking for is one that gets smaller and smaller as 'x' grows.
That has to be a fraction that starts out less than '1', so the more times
you multiply it by itself, the smaller it gets.
The only tricky trick here is to know how to handle the ones with the exponent -x .
Where you see a fraction raised to the -x power, you can flip the fraction over
and then change the power to just plain 'x'.
When you go down the list and examine each one, you find that the only one
that keeps getting smaller as 'x' keeps getting bigger is the 3rd one down.
