Which of the following is the best example of a symbiotic relationship?

Certain breeds of fish will often clean bigger fish. The cleaner fish gets food, the bigger fish gets a cleaning.
or
A foolish man decides to go swimming in shark-infested waters. As a result, he is bitten by a shark looking for food.
or
A pair of birds fight over a piece of food. The dominant bird wins.
or
The climate changes. As a result, hundreds of species become extinct and the land turns into a desert.
?