A program is estimated to take $100$ clock cycles. If your system clock speed is $40$ MHz$,$ then this program's execution time will be $\_\_\_\_\_\_\_\_.($Please make two selections: $(1)$ the Greek lettered prefix along with the unit of time $($i$.$e$.$ second$),$ and $(2)$ the numerical value$)$ This is a multi$-$select question.

Question $5$ options:

$0\mathrm{.}25$

ms

ns

$2\mathrm{.}5$

us

$25$