OE
In this question, it is hard to select an answer for either blank in isolation. The “brief survey,” like any survey, could in fact be surprisingly “distorted,” “objective,” or “comprehensive.” The contrast in the following sentence with “longer treatments” suggests that the brevity of the survey is important. Of the three choices, only “comprehensive” is particularly unexpected of a brief survey. Provisionally accepting “comprehensive” makes it easier to analyze the second blank. If the short survey is surprisingly comprehensive, then longer treatments may not convey any useful additional information, making them “redundant.” Longer works are unlikely to seem “pithy” in comparison to shorter ones, and “premature” makes little sense in this context. Reading the sentence again with “comprehensive” and “redundant” filling the blanks confirms that these two choices result in a coherent whole.
Thus, the correct answer is comprehensive (Choice C) and redundant (Choice D).