Context: Below is a food web from a tundra ecosystem in Nunavut, a territory in Northern Canada.
A food web models how the matter eaten by organisms moves through an ecosystem. The arrows in a food web represent how matter moves between organisms in an ecosystem.
Based on the arrows, which of the following organisms is a decomposer?
A. mushroom
B. lichen
Determine the correct option from the provided choices and reply with its corresponding letter.

A