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I had a look here: http://www.askoxford.com/concise_oed/thesis?view=uk
Good enough for me
• noun (pl. theses /theeseez/) 1 a statement or theory put forward to be maintained or proved. 2 a long essay or dissertation involving personal research, written as part of a university degree.
— ORIGIN Greek, ‘placing, a proposition’.
Austin Charles- UNIS
Anonymous Or, thesis' if you are saying that something belongs to a particular thesis.You are referring to the possessive, not the plural.
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