Questions 18 to 20 are based on the following passage. At the end of the passage, you will be given 15 seconds to answer the questions. Now listen to the passage.

What does the well-known joke imply

A．Remarriages often end up in failure.

B．Children are unhappy in the new family.

C．The mother can’t get along with her stepchildren.

D．Remarriages cause new troubles in the household.

从下列数据库分割条件中，选出用于抵御跟踪器攻击和抵御对线性系统攻击的一项。（）。

A、每个分割区G有g=|G|记录，其中g=0或g>=n，且g为偶数，

B、记录必须成对地加入G或从G中删除

C、查询集虚报口各个分割区，如果查询含有一个以上记录的统计信息是从m各分割区G1，G2，……Gm中每一个分割区而来的，则统计信息g（G1VG2V……VGm）是允许发布

D、记录必须不对地加入G或从G中删除