TEXT D Lichens can be spectacular for anyone who cares to look, but few people take the trouble. Often modestly colored, and seemingly two-dimensional as they cling to whatever surface they find, they grow in background - as though designed to be ignored. Yet they hold a special fascination for botanists, partly because they present mysteries still to be solved and partly because they do so many things so well. No casual observer ’of a lichen would ever suspect that it was a composite of interacting life forms. This seemingly uncomplicated lichen is actually composed of a fungus and a colony of algae (or blue-green algae, which some scientists now consider to be bacteria). A few species even include all three of these diverse forms of life. A complete lichen is strikingly different from its separated partners in both appearance and biochemistry-many produce unique compounds which cannot be made by the component organisms alone. Lichens grow in almost every natural habitat imaginable, from deserts to tropical rain forests-even on the backs of certain beetles in New Guinea, and i0side rocks (along with algae ) in the otherwise barren dry valleys of Antarctica. Many species can tolerate extreme heat, cold. or dryness. Very few, however, can survive heavy air pollution, and many live only where the air is very clean. The disappearance of lichens from an area gives warning of a threatened environment. The author states that lichens grow "as though designed to be ignored" because they are ______.

A. not totally understood by botanists
B. troublesome to collect for the purposes of study
C. uncomplicated in their internal structure
D. not easily noticed by observers