使用逗号分隔枚举中的项目。

Example: Old McDonald had a pig, a dog, a cow, a horse.

“and”之前的逗号是可选的。

Example:
 Old McDonald had a pig, a dog, a cow and a horse.
 Old McDonald had a pig, a dog, a cow, and a horse.

如果两个项目是同一个unit，请不要在“and”之前使用逗号 (“Ham and eggs” as a dish is a unit and should therefore not be separated by a comma.)

Example:
 Old McDonald had soup, ham and eggs and apple pie for dinner.
 Old McDonald had soup, ham and eggs, and apple pie for dinner.

如果枚举中的所有项都以“and”, “or”, “nor”分隔，请不要使用逗号 

Example:
 Old McDonald had a pig and a dog and a cow and a horse.
 Old McDonald had a pig or a dog or a cow or a horse.
 Old McDonald neither had a pig nor a dog nor a cow nor a horse.

therefore [´ðeəfɔ:]   ad.&conj.因此；所以   (初中英语单词)