Question 425503
(a) is not true
(b) is true
This is a cause and effect problem. I like to think
of something much easier, like:
(a) Is plant growth a function of rainfall
or
(b) Is rainfall a function of plant growth? 
--------------------------------
You can write it mathematically like:
F(rainfall) = plant growth
This says plant growth is caused by rainfall
If I said
F(plant growth) = rainfall
Now I'm saying rainfall is caused by plant growth
--------------------------------------
When you make a graph, you put the cause on the horizontal axis
and you put the result on the vertical axis.
So, in your problem:
Number of minutes is on the horizontal (cause)
Long distance rates are on the vertical (result)