题目分析：
本题主要考查了关于“平行”和措辞的一些用法。

选项分析：

A选项：并列连词and需要连接两个相同的成分。本选项and身前是介词短语with higher-than-average blood pressure，身后是句子their families have a history of high blood pressure。两者无法平行。

B选项：Correct. 本选项在语法和逻辑上均是正确的。whose families have a history of high blood pressure是定语从句，其先行词为young people。这里的跳跃修饰是没有问题的。

C选项：本选项错误同(A)。

D选项：分词短语running in them是赘余的。families have a history of high blood pressure已经表示在家庭中有高血压的情况了。

E选项：介词短语with higher-than-average blood pressure和with a history of high blood pressure running in their family之间缺少连词连接，正确方式为：
young people with higher-than-average blood pressure and with a history of high blood pressure running in their family are more likely than others to develop a severe form of the condition. 

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