Someone wrote in a letter: "I saw the platypus egg in Manchester a few years ago, so I ignorantly mocked this stupid opinion that mammals lay eggs, and now it is finally confirmed",. It can be seen from the letter that the writer had previously thought wrongly? ['The platypus does not lay eggs.', 'Some mammals do not lay eggs.', 'Platypus is not a mammal.', 'Only mammals are mammals.']