A.I. Explanation:

Option 1: "There are too many species to count them all."

Explanation: While it is true that there are many species, the text emphasizes that the difficulty in establishing a number is due to the constant changes in species, not merely their quantity. This option oversimplifies the issue and does not address the dynamic nature of species populations.

Option 2: "The number of species is constantly changing."

Explanation: This option accurately reflects the main idea of the text, which states that new species are continually appearing while others evolve or become extinct. This constant change makes it impossible to establish a definitive number of species, aligning perfectly with the text's explanation of the complexities involved.

Option 3: "Currently, more species are appearing than are becoming extinct."

Explanation: This statement is incorrect as the text indicates that the trend has reversed; currently, more species are becoming extinct than are appearing. Therefore, this option contradicts the information provided in the text.

Option 4: "Human action has upset the balance of the natural environment."

Explanation: Although the text mentions human activity affecting the ecosystem, this option does not directly address the question of why it is difficult to establish the number of species. It provides a relevant fact but fails to connect it to the main issue of changing species numbers.