Consider a process executing on an operating system that uses demand paging. The average time for a memory access in the system is $M$ units if the corresponding memory page is available in memory, and $D$ units if the memory access causes a page fault. It has been experimentally measured that the average time taken for a memory access in the process is $X$ units.
Which one of the following is the correct expression for the page fault rate experienced by the process.
- $(D-M) / X-M)$
- $(X-M) / D-M)$
- $(D-X) / D-M)$
- $(X-M) / D-X)$