题目选项分析

Humans have been damaging the environment for centuries by overcutting trees and farming too intensively, and though some protective measures, like the establishment of national forests and wildlife sanctuaries, having been taken decades ago, great increases in population and in the intensity of industrialization are causing a worldwide ecological crisis.

Humans have been damaging the environment for centuries by overcutting trees and farming too intensively, and though some protective measures, such as the establishment of national forests and wildlife sanctuaries, were taken decades ago, great increases in population and in the intensity of industrialization are causing a worldwide ecological crisis.