20.  The purpose of non-REM sleep is even more ______, The new experiments, such as those described for the first time at a recent meeting of the Society for Sleep Research in Minneapolis, suggest fascinating explanations for the purpose of non-REM sleep.
【正确答案】 C
【答案解析】 同义复现。开头说的the purpose of non-REM sleep和结尾的the purpose of non-REM sleep遥相呼应，而在后文的fascinating就提示空格应填与其同义的词，故答案为mysterious(神秘的，为之着迷的)。