Extent: (n) indicating how great, important or serious a situation or difficulty is. Which: (pron) You use which at the beginning of a relative clause when specifying the thing that you are talking about or when giving more information about it AND You use which to refer back to an idea or situation expressed in a previous sentence or sentences, especially when you want to give your opinion about it. (from http://dictionary.reverso.net/english-cobuild/which ) A similar phrase would be "the degree that".