As a Fish Forces Water Across Its Gill, the Blood

As a fish forces water across its gill, the blood flowing through the gill

A) picks up O₂ over the whole route of exchange.
B) picks up O₂ during the first half of the exchange path, then equilibrates.
C) picks up O₂ over the second half of the exchange route only.
D) removes O₂ from the gill and then releases most of its O₂ back to the gill.
E) releases O₂ over the whole route of exchange.