If the Seawater Available to an Estuary Has a Salinity

If the seawater available to an estuary has a salinity of 30‰ and the seaward-flowing mixed water has a salinity of 20‰ at the estuary entrance, the net volume transport seaward is ________ units per day, if the river input is 10 units per day.

A) 5
B) 10
C) 20
D) 30
E) 50