请各位帮我分析四个句子的语法,万分感谢! The lecture is in class NO.2,and it begins at 9:00. The lecture is in class NO.2 and it begins at 9:00. The lecture is in class NO.2 and begins at 9:00. The lecture is in class NO.2,and begins at 9:00. 以上四个句子有哪个语法不对吗?为什么不对?正确的句子,为什么正确?能不能从逗号与and的用法讲解一下? 万分感谢!我会追问到底的,请诸位尽量详尽,我就是想写出正确的句子.