参考答案：D

解析：比较结构。根据原文“The research paper was very brief, but still it was better than I expected.”，译文为：这篇论文很简短，但仍然比我预想的好。此题关键在于理解brief的词义和转折连词but之后的比较级结构。brief的意思是：简洁的，简短的。a选项“The paper came out as short as I expected．”，这篇论文写出后和我预想的一样短。b选项“Actually the paper was long enough to cover it．”，实际上这篇论文足够长，能涵盖所有要点。C选项“The paper was short enough to present．”，这篇论文很短，足够做报告。d选项“The paper turned out better than I had expected．”，这篇论文结果比我预想的要好。