One type of fish is named salmon(大马哈鱼). There are many different kinds of salmon, but almost all of them mate(交配) in a very strange way. They are born in small, freshwater rivers. They live in the river for a year, and then swim downstream to a bigger river and eventually to the ocean very quickly. While they live in the ocean, salmon are silver coloured.
After several years at sea, the salmon grow very long and heavy. Then, as if by magic, they all begin to swim home. Each salmon somehow remembers where it was born and the very same area in the small river.
It is a long, difficult journey home. Once the salmon enter fresh, they stop eating. They also change colours. Some salmon turn red, other brown and grey, and some turn pink. Some salmon grow a large bulge on their back, called a hump. These salmon fight to swim upstream against the current of sometimes very to avoid bears, birds, and fishermen
Finally, the salmon that survive mate in the same river in which they were born. Then, after all that work, they die! These salmon still play an important role. Their dead bodies help to provide nutrition(营养) to the animals that live in and around the river When their eggs hatch, the cycle will start again.
Which of the following do salmon NOT do when they reenter fresh water?

A．They change colours.

B．They stop eating.

C．They grow bulges on their backs.

D．They grow teeth.

Inferring from the passage, why do salmon eat a lot while they are in the ocean? Because_______

A．they need to have enough energy to return home

B．they never stop swimming

C．they are afraid of bears

D．the different water makes them hungry

Salmon return to fresh water in order to _______.

A．mate

B．provide nutrition to other animals

C．mate and provide nutrition to other animals

D．make a difficult journey