In this task, you're given an article and an answer. Your task is to generate the fill-in-the-blank question for this answer based on the given article. The blanks are always at the end of the question, and can be filled with more than one word or phrase.

Article: Whales live in the ocean, but they are not fish. They are warm-blooded mammals and must have air to breathe. The babies are born alive and drink milk from the mother's body. It took millions of years for the whale to develop as it is today. Every spring groups of whales swim hundreds of kilometers to warm places to bear their young. Each group goes to the same place every year. The mothers take very good care of their babies. The fully grown whales are very gentle and playful with each other, and with humans. They "talk" to each other with a high noise that sounds beautiful. This"talking" can be heard for more than 300 kilometers in open waters. Yet humans kill over a hundred whales a day. They kill them to make whale oils and a lot of other things. Killing them is a modern business with modern machines and even harpoons( ) that explode( ) inside the whales. A whale dies very slowly and in great pain. Some ships kill every whale they can find, even the mothers and babies. Then there are no young whales to grow up or to bear more young.
Answer: whales are fish
A commonly mistaken belief is that _ .