Which of the following statements is an example of a harmful action by fungus?

a. If bread is left out and exposed to air too long, it can grow mold.
b. If you add yeast to a drink with sugar, it can create bubbles in the form of carbonation.
c. If mushrooms grow on a dead organism, they can break them down recycling nutrients back into the ground.
d. If fungal spores are floating in the air, they can eventually fall to the ground creating new mushrooms.