用B时，前面是逗号，先行词是next Saturday.非限制性定语从句。
The Smiths will move into this new house next Saturday,by which time it will be completely finished.
当前面是句号时，是两个句子。用A
The Smiths will move into this new house next Saturday. By that time it will be completely finished.
或者用and连接两个并列句：
The Smiths will move into this new house next Saturday and by that time it will be completely finished.
选B
一个很简单的方法，这样的非限定性定语从句只要前半句出现“，” 那肯定选which而不用that了~~~~
具体就是修饰物时用which，人用who
缺少介词 在which前加上相应介词就好了~~~
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