Otters spend most of their time in the water. Over time otters have developed a coat with soft underfur to trap warm air near their skin and an outer layer of longer hair that keeps them dry underwater. Which of the following describes how otters developed this type of fur coat?

The otters adapted gradually over many generations.
The otters adapted gradually over many generations.

Cold weather triggers the underfur to grow each winter.
Cold weather triggers the underfur to grow each winter.

The otters mated with other aquatic mammal species having thick fur.
The otters mated with other aquatic mammal species having thick fur.

The significant amount of time otters spend grooming their fur encouraged this trait.
The significant amount of time otters spend grooming their fur encouraged this trait.