Make use of the current perfect tense to advise that a little something has transpired greater than when prior to now and should be continuing; By way of example, "In the last 20 years, researchers have generally concentrated on the consequences of posttraumatic stress disorder." ♦ 把每个关键词隔离开，并在对应的主题内搜索这个关键词或者关键词的同义词； A fast note https://hylistings.com/story12299229/%E7%BE%8E%E5%9B%BDliterature-review%E4%BB%A3%E5%86%99-options