Aphids Pierce the Tissues of Plants with Their Tubular Mouthparts

Aphids pierce the tissues of plants with their tubular mouthparts and suck out the phloem juices. Because this food is mostly sugar water with a low concentration of protein, the aphids have to process large volumes of it to obtain sufficient protein. Most of the excess sugar and water passes right through the aphids' digestive tracts and is defecated as "honeydew," which is actively sought out and eaten by some ants. The ants will even "milk" honeydew by stroking the aphids. In return, ants protect their aphids from predators, even going so far as taking them inside overnight. The relationship between the aphids and ants is called

A) mutualism.
B) parasitism.
C) competition.
D) predation.