单项选择题

C

Trees have a spectacular survival record. Over a period of more than 400 million years, they have evolved as the tallest, most massive, and longest-lived organisms ever to inhabit the Earth. Yet trees lack a means of defense that almost every animal has: trees cannot move away from destructive forces. Because they cannot move, all types of living and nonliving enemies—fire, storms, microorganisms, insects, other animals and, later, humans—have wounded them throughout their history. Trees have survived, because their evolution has made them into highly compartmented organisms. In that respect trees are radically different from animals. Fundamentally, animals heal: they preserve their life by making billions of repairs, installing new cells or rejuvenated cells in the positions of old ones. Trees cannot heal: they make no repairs. Instead, they defend themselves from the consequences of injury and infection by walling off the damage. At the same time they put new cells in new positions; in effect, they grow a new tree over the old one every year. The most obvious results of the process are growth rings, which are visible on the cross section of a trunk, a root, to a branch.

In the passage, the writer tries to explain ______.