Tina and Sarah are from cuba.（改为否定句）为什么不能写成no,they,aren't

No,they aren't 不是改成否定句啊,是作are they from cuba的否定回答。改为否定句应该是在原句里面加上否定词,这句话应该改成否定句应该是Tina and Sarah are not from cuba。