There is mounting evidence that the frequency and magnitude of landsliding is changing in many parts of the world in response to climate change. This is not surprising, given that precipitation is one of the two external triggering mechanisms--the other being seismic activity--involved in the formation of landslides. Evidence from the past clearly indicates that cycles of elevated landslide activity have been followed by cycles of low activity, and that these are correlated with climate fluctuations over a variety of timescales.
What sets current changes in landslide activity apart is the likely influence of anthropogenic [i.e., human-caused] factors, either acting alone or in concert with climate, which can further modify the process of landsliding and the nature of ecosystem responses. Among these factors, deforestation and land-use change have the potential to influence the frequency and magnitude of landsliding because of their direct effects on vegetation attributes that influence slope stability. The extent and conditions under which mountain ecosystems are resilient to these changes--that is, the amount of disturbance they can absorb before changing into states with different structure and function--are not known. Addressing this issue is crucial for the long-term conservation of mountainscapes.

3. The author of the passage suggests which of the following about the role of human factors in landsliding activity?
A. Human factors can intensify or alter the effects of climate change on landsliding activity.
B. It is likely that human factors affect landsliding activity more than climate change does.
C. Until recently, human factors did not have much impact on landsliding activity.
这一题为什么选AC？C选项为什么正确呀？