Practice Test: Reading Comprehension - 31 | 100 RCs for GMAT PDF Download

The question asks for the purpose of the passage. The passage focuses on an experiment designed to challenge an existing theory about bacteria. The correct answer must take the entirety of the passage into account without misrepresenting its focus.

(A) The “innovative technique” is the replica plating method discussed in the third paragraph. This technique was used to support the scientific hypothesis of Fildes and Whitaker. Thus, the passage did not need to “defend [Fildes and Whitaker’s] scientific hypothesis from attack by [the] innovative technique.” In addition, the passage did not even defend the theory that Fildes and Whitaker were challenging.

(B) The first sentence of the passage makes it clear that the focus will be to “challenge the theory that strains of bacteria can be “trained” to mutate,” not to describe a process by which such training was accomplished.

(C) CORRECT: The second and third paragraphs discuss the experiments performed by Fildes and Whitaker that were designed to test the established theory, mentioned in the first paragraph, that “strains of bacteria can be “trained” to mutate.” This answer choice summarizes the entire passage.

(D) Fildes and Whitaker argue against a theory, not an established protocol. There is no mention in the passage that a particular protocol is outdated.

(E) The passage does not state that either Fildes and Whitaker’s theory, or the theory they were challenging was questionable. The passage describes, but does not “challenge,” several scientific techniques.

This is a specific detail question that asks what the passage states about the use of liquid culture. The use of liquid culture is discussed in the second paragraph.

(A) Both agar culture and tryptophan are mentioned in the passage, but are discussed separately: bacteria were grown in an agar culture, and certain bacteria require tryptophan in order to reproduce. The passage does not state that the agar culture contained tryptophan.

(B) Although plating is discussed in the second and third paragraphs, the difficulty of plating was not mentioned.

(C) The passage did not mention the number of bacteria that could be supported by the agar culture.

(D) CORRECT: According to the second paragraph of the passage, "because of the possibility that the lack of uniformity of the agar cultures had in fact trained mutant strains, they [Fildes and Whitaker] conducted similar experiments with liquid cultures..." Thus, the liquid culture is a more consistent, or uniform, medium.

(E) In the third paragraph, it is stated that “Fildes and Whitaker used an innovative plating technique using pile fabrics…” The plating process and the results demonstrated by this method are subsequently explained. The liquid culture experiments are discussed separately, in the second paragraph. The passage did not state that the agar culture did not adhere well to pile fabric.

The replica plating method is discussed in the third paragraph. The first two sentences describe the process, while the third sentence explains what was learned by using this method. The correct inference about the effectiveness of the replica plating method will be a virtual rephrasing of the facts presented in the third paragraph.

(A) We cannot infer that the replica plating method allows researchers to determine the relative sizes of different populations of bacteria. The population sizes of various bacteria populations were never discussed.

(B) CORRECT: The last sentence of the passage states that “the locations of the mutant strains of bacteria on the replica plates were identical to the locations of the mutant strains on the original agar plate,” citing this as proof that “the mutants were in fact genetically present or preadapted.” In other words, the mutant strains did not develop after transfer from the original agar plate.

(C) The only type of bacteria mentioned in the passage was bacteria typhosum, so we cannot infer that the replica plating method eliminates the possibility that the agar culture was contaminated by a different type of bacteria.

(D) The role of tryptophan is discussed in the first and second paragraphs. This answer choice goes too far by asserting that “no tryptophan was present in the original agar culture.” We do not have enough information to make that inference.

(E) According to the first paragraph, “the necessary metabolites for bacterial reproduction” are defined as the substances “necessary for their (bacteria’s) regular function.” In the case of bacteria typhosum, that substance is tryptophan. We do not have enough information to infer that the replica plating method establishes that the original agar culture contained tryptophan. If fact, the presence of mutant strains of bacteria on the replica plates would contradict this statement rather than support it.