染色体是细胞内遗传物质的主要载体．下图为细胞内染色体的组成关系图解．请据图回答问题：

（1）生物体内a的种类约有____ 　种，其结构通式为　____  ．A是由a经____  反应形成的，形成肽键的结构简式是____ ．

（2）细胞中b的种类有____  种，分子中____  不同，决定了b的种类不同．B主要分布在____ 内．

如图是光源S经平面镜反射的两条光线，请画图确定光源位置，并完成光路图．

    Charlie Chaplin was considered to be one of the most popular ____ (star) in the early days of films. He brightened the lives of people throughout the world ____(use) his perfect acting. Charlie was born in a poor family and lived ____ hard life in his childhood. His parents were both poor hall performers,____ didn't make much money. Therefore, at a very young age, Charlie had to learn ____ (sing) and dance. By his teens, his subtle acting had become so entertaining ____ no one in England was ever bored watching him mime and act the fool doing ordinary everyday tasks. In 1972, he won a special Oscar ____his outstanding work in the films that he wrote, directed, produced, and starred in. He made people laugh even when they felt depressed. And no one has done ____ (well) than him up to now.

    Charlie lived in England and the USA but ____(spend) his last years in Switzerland, where he died of ____ (nature) causes on December 25, 1977.

一颗牙齿的结构包括____和____。