Air Raid Inc. designed $20$ air$-$conditioning systems designed for use by astronauts in the International Space Station. All of them were operated for $1,000$ hours at NASA's test facility. Out of all the systems $2$ of them failed during the test. One of them after $200$ hours and the other after $600$ hours.

What is the Per Unit Failure Rate?

A $,0\mathrm{.}000106$ failure$/$unit hour

B $,0\mathrm{.}000504$ failure$/$unit hour

C $0\mathrm{.}250346$ failure$/$unit hour

D $,0\mathrm{.}0000001$ failure$/$unit hour

E $2$ failures$/$unit hour