English 1b3, You questions seem to be involved with dependent and independent clauses, as well as punctuations. I picked this one as the correct answer. Also the following link will help you clear up some, if not all, your questions.
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English 1b3I've heard some scary stories from Mum's friends, one of whom had a cousin murdered for speaking rudely
English 1b3couldn't one argue that it is a summative modifier?No, because it isn't a summative modifier. You are not re-explaining or redefining Mum's friends or scary stories; you are adding new information. Besides, a summative modifier at that point in the sentence would have to be plural to go with friends or with stories, an