下列这些水中生活的动物中，哪一组都是无脊椎动物？(    )

    In America，primary school lasts six years. There are about twenty students in one class. They are in different groups. Different groups may have different textbooks. The whole grade graduate(毕业)at the same time，but the students learn different knowledge.

    Primary school students don't have schoolbags. Their books are in the school. The students usually study reading，arithmetic(算术)，language，science，music，art and PE. The performances(成绩)show three grades. G means good. S means satisfaction. N means you need to work hard.