Throughput Accounting Ratio

Hi
Can you please explain the following question to me?

ABC Company uses throughput accounting. Machine time is the current binding constraint on production output and the management are looking for ways to increase the throughput accounting (TA) ratio for a product that the machine is used to manufacture.

Which of the following will have no effect on the TA ration?

a. increasing the selling price of the product
b. obtaining a lower purchase price for materials for the product
c. reducing factory costs
d. reducing the machine time per unit to make the product

The answer is d.
The answer states that reducing the machine time per unit will increase throughput per hour and factory costs per hour by the same proportion, leaving the TA ratio unchanged.

But I do not understand how the factory cost per hour gets reduced when the machine time per unit is reduced.

Please explain.
Thank you very much!