Mary likes to invent things. She creates a new language and teaches it to her friend, Jean. Mary and Jean like to talk to each other in their own language because they think no one understands them and it is funny. When they go shopping, they say things like: "Look! He is fat.” “This CD is awful." "I want to eat free hamburgers.”A clerk comes but he can't understand what they say. He asked them: “May I help you?”Mary and Jean just laugh. What fun it is to create something new.

下列材料为人类染色体数目确定的科学历程：

①20世纪初，细胞学家提出人体细胞染色体数目为47条，其中一条为“副”染色体，即现知的X染色体。

 ②1923年科学家又发现了仅存在于男性细胞中的Y染色体，得出了人体细胞共有48条染色体的结论，并分别用“46+XX”和“46+XY”表示女性和男性体细胞中染色体的组成。

③1955年科学家最终确定了人类染色体的正确数目，即人体细胞共有46条(23对)染色体，其中包括两条性染色体：女性为XX，男性为XY。