A research team examined a long-term owl roost. Owls prey on small mammals and the excreted remains of those meals that accumulated over the time, provide us an insight into the composition and structure of small mammals over the past 'millennia. The research suggested that when the Earth went through a period of rapid warming about 13,000 years ago, the small mammal community was stable and resilient. But, 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. On the basis of the above passage, the following assumptions have been made : 1. Global warming is a frequently occurring natural phenomenon. 2. The impending global warming will not adversely affect small mammals. 3. Humans are responsible for the loss of the Earth's natural resilience. Which of the above assumptions is/are valid?

Analysis of the statements:

1. Global warming is a frequently occurring natural phenomenon. The passage mentions only one instance of rapid warming about 13,000 years ago, which was natural. It does not provide enough information to conclude that such phenomena are "frequently occurring." Therefore, this assumption is not valid.

2. The impending global warming will not adversely affect small mammals. The passage states that while small mammals were resilient during past natural warming, "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 implies that modern ecosystems, due to human impact, will be adversely affected by future warming, unlike in the past. Therefore, this assumption is not valid; in fact, the passage suggests the opposite.

3. Humans are responsible for the loss of the Earth's natural resilience. The passage explicitly states that "human-made changes to the environment had caused an enormous drop in biomass and energy flow," and this decline means "modern ecosystems are not adapting as easily as they did in the past." This directly links human actions to a reduced capacity of ecosystems to adapt, which is a loss of natural resilience compared to the past. Therefore, this assumption is valid.

Based on the analysis, only assumption 3 is valid.

The final answer is B) 3 only.