Read the passage. North American river otters are a species whose population can indicate the degree of health in the surrounding environment. The otters experienced a sharp decrease in numbers in the 1900s. This was due to fur trapping and pollution. They are not considered endangered today, though.

Which sentence could be added to the passage to maintain its formal style?

The otter is on the endangered species list.

Fur trapping and pollution are legal today.

If people would leave otters alone, they would be healthy.

Through conservation efforts, the otter population has recovered.