Explain how the Linux kernel variables $HZ$ and jiffies can be used to determine the number of seconds the system has been running since it was booted.

You can use an example e$.$g$.$ Given $5HZ$ and $20$ jiffies, here is how I can determine how many seconds have elapsed since my system started running. Then explain your reasoning behind your process and show your calculation $(5$ points$)$

Rank the following storage systems from slowest to fastest:

a$.$ Hard$-$disk drives

b$.$ Registers

c$.$ Optical disk

d$.$ Main memory

e$.$ Nonvolatile memory

f$.$ Magnetic tapes

g$.$ Cache