某同学从一种未知的无色溶液中检验出Ba²⁺、Ag⁺，同时溶液的酸性很强，他准备继续检验溶液中是否含有Cu²⁺、Cl^﹣、CO₃^2﹣、S^2﹣、K⁺、Al³⁺、SO₄^2﹣、SO₃^2﹣、NO₃^﹣等离子．显然，有些离子还必须检验，你认为还必须检验的离子是____ ．

如图1（注：与图2完全相同），二次函数y= $\frac{4}{3}$ x²+bx+c的图象与x轴交于A（3，0），B（﹣1，0）两点，与y轴交于点C．

    Many people believe that language belongs to human beings. However, cats have developed a language not for each other, but for the human beings who have them as pets.

    When communicating with each other, cats “talk” with a system of signals. Their tails, rather than any kind of “speech”, act as an important way to express themselves. They also touch each other to express s their feelings. With other cats, they will use their voices only to express pain. Unbelievable, all of that changes when a human walks into the room. Cats use many different kinds of vocal (声音的) expressions when they communicate with a person. Since these vocal expressions are not used to communicate with other cats, it seems that cats developed this “language” to communicate with their human owners.

    This fact is shown more clearly when watching rooms that have only one cat and those with several cats. An only cat is usually very vocal, since the only creature(动物)around with whom the cat can communicate is its owner. Cats with other cats, though, are much quieter. If they want to have a conversation, they only need go to other cats and communicate in their natural way.

    Since cats learned to meow(喵喵叫)for the only purpose of communicating with human owners should take the time to learn what their different meows mean. If an owner knows, to name just a few examples, which meow means the cat is hungry, which means the cat wants to be petted, and which means the cat wants to have a little “conversation”, the relationship between cat and owner will be closer.