英语作文人工批改：namely的前后要用逗号吗

namely（即，就是）前面通常用逗号，后面不能直接用逗号，即不用 namely, 请根据以下例句体会具体用法（摘自张道真主编《现代英语用法词典》修订版namely词条)：

Only one person can do the job, namely you.

The means of production, namely factories, mines and land should be owned by the state.

Three students were mentioned, namely John, Sarah and Sylvia.

更多例句如:

Only one boy was absent, namely Harry. 只有一个男孩儿缺席，就是哈里。

She made a suggestion, namely that the student not be admitted. 她提出一个建议，即不录取那个学生。

They brought lunch, namely sandwiches, chips, and soda. 他们带来了午饭，即三明治、薯条和汽水。

Three students were mentioned, namely John, Sarah and Sylvia. 有三名学生被提到，即约翰、萨拉和西尔维娅。

I shall now turn to the fourth factor, namely markets. 我现在讲第四个因素，也就是市场因素。

I would like to buy a new car, namely a Mercedes. 我很想买一辆新汽车，也就是买一辆梅塞德斯牌汽车。

The other change, namely the increase in electronic equipment, has slowed down. 其他变化，即电器的增长，已经开始缓慢下来。

但在某些特殊情况下也可能后接逗号(比如后一个疑问句时):

I have a question; namely, should we sell the property or not? 我有一个问题，那就是我们要不要变卖财产？