The acacia tree is protected from other insects by a certain species of ant. The ants, in turn, feed on a sweet secretion produced by the tree. This type of nutritional relationship is known as

a) mutualism
b)parasitism
c)commensalism
d)saprophytism

step 1: nitrogen fixation- conversion of atmospheric no₂ into forms useful to plants by lightning, bacteria, and cyanobacteria.

step 2: nitrification-process of converting ammonia to nitrite or nitrate.

step 3: assimilation-plant roots absorb new form of nitrogen.

step 4: ammonification-conversion of organic nitrogen to ammonia by bacteria involved in decomposition.

step 5: denitrification-bacteria turns the decomposed matter back into n₂o, which is released into atmosphere; repeat cycle.

i think the answer is a.keratin is the pigment responsible for skin color.

is it multiple choice?