Fill in the blanks.

$($a$)\_\_\_\_\_\_$ causes are also called natural causes, noise and random causes. $($b$)\_\_\_\_$ cause variation, on the other hand, is the unexpected variation in the process.

Group of answer choices

$($a$)$ common $($b$)$ assignable

$($a$)$ special $($b$)$ assignable

$($a$)$ assignable $($b$)$ common

None of the above