$($i$)$ Which is the correct definition for the $2$UH:

$($a$)$ the quick response runoff due to a half unit depth of net rain occurring per

hour

uniformly in space and time over a duration of $2$ hours.

$($b$)$ the gross runoff due to a unit depth of gross rain occurring uniformly in space

and

time over a duration of $2$ hours.

$($c$)$ the quick response runoff due to $1$ unit depth of net rain occurring per hour

uniformly in space and time over a duration of $2$ hour.