and做连词时,前面一定要加逗号吗?

题目

and做连词时,前面一定要加逗号吗?
比如
I have a book,a pencil,and a bike.这里逗号能省略么?是一定要有,还是可有可无,或压根不该有?
如果and连接分句的话,之前的逗号能省略么?

答案

例：I have a book and a pencil.I have a book,a pensil,an a piece of paper.She is kind and beautiful.She is kind,smart,and beautiful.看出来了吗,前面有逗号,则后面例举的时候,在and前也要....