【单选题】

Observe the dilemma of the fungus: it is a plant, but it possesses no chlorophyl. While all other plants put the sun’s energy to work for them combining the nutrients of ground and air into body structure, the chlorophylless fungus must look elsewhere for an energy supply. It finds it in those other plants which, having received their energy free from the sun, relinquish it at some point in their cycle either to other animals (like us humans) or to fungi.

In this search for energy the fungus has become the earth’ s major source of rot and decay. Wherever you see mold forming on a piece of bread, or a pile of leaves turning to compost, or a blown-down tree becoming pulp on the ground, you are watching a fungus eating. Without fungus action the earth would be piled high with the dead plant life of past centuries. In fact, certain plants which contain resins that are toxic to fungi will last indefinitely. Specimens of the redwood, for instance, can still be found resting on the forest floor centuries after having been blown down.
The author uses the word "dilemma" to indicate that ______.
A、the fungus is both helpful and harmful in its effects
B、no one understands how a fungus lives
C、fungi are not really plants
D、the fungus seems to have its own biological laws