Correct Option (B)
Assumption 3 is valid. The passage explicitly states that "from the last quarter of the nineteenth century, human-made changes to the environment had caused an enormous drop in biomass and energy flow. This dramatic decline in energy flow means modern ecosystems are not adapting as easily as they did in the past." This directly implies that human activities are responsible for the diminished adaptive capacity, or natural resilience, of Earth's ecosystems.
Incorrect Options:
- Assumption 1: "Global warming is a frequently occurring natural phenomenon." The passage mentions a single instance of rapid warming approximately 13,000 years ago, which was a natural event. However, it does not provide information to conclude that global warming is a "frequently occurring" natural phenomenon in general.
- Assumption 2: "The impending global warming will not adversely affect small mammals." The passage suggests the opposite. It highlights that while small mammal communities were resilient to natural warming in the past, modern ecosystems, due to human-made changes, are now struggling to adapt. This indicates that current environmental changes, including global warming, are likely to have adverse effects.