Mutualism- in which two different organisms interact, and each organism benefits each other (think "positive, positive"; or " +,+ ").
An example would be Oxpeckers and zebras or rhinos - In this relationship, the oxpecker (a bird) lives on the zebra or rhino, sustaining itself by eating all of the bugs and parasites on the animal.
The bird benefits by having a readily available source of food.
The zebra or rhino benefits from having the bugs removed. Also, when there is a danger to the zebra or the rhino, the oxpecker flies high and makes much noise in order to alert nearby animals to the impending danger (i.e. a predator).
Read more at http://examples.yourdictionary.com/mutualism-examples.html#3LtWEwmHBJ53KFbb.99
Your answer is C: A rapid increase in the number of related species
The magnetic field of a bar magnet is the strongest at either pole of the magnet. It is equally strong at the North Pole when compared with the south pole. So I would say the answer is A.
