Yes, it's B. The "...it is ______ that..." part clearly indicates that some "thing" must fill the space. B. "your being overweight" is a noun phrase (=a thing), so it fits perfectly. A isn't even grammatically correct; C is part of a sentence and not a single "thing"; D is a complete sentence, and again does not stand as a single "thing".

谢谢大家的帮助！

B 简单点说，这里仅需要一个overweight就够了。B在此等同于overweight。

选B 后半句是强调句型it is...that... 强调句型的特点是把it和is去掉后那句话仍然是完整的，所以后半句把it和is去掉后，那句话缺少主语，而作主语的只能是名词或名词性短语（v-ing或to do）,所以选B。 你也可以把答案带到题目中去检查，把B带到题目中就是it is your being overweight that causes your illness,把it和is去掉的话就是your being overweight causes your illness,是完整的没有语法错误的一句话，所以选它不用怀疑了。

选B。 不管你相不相信，是你体重超重引发了疾病。 语法上来说，这里缺少一个表语，而它同时用来做后面定语从句中的逻辑主语。而学校里的老师会告诉你这是典型的强调句。因为后面是个定语从句，所以缺少的必须是个名词性质的词。而A语法上就错误的，C和D都是一个句子，而B your being overweight是you are overweight的分词形式，语法上就和一个名词一样。