The English physician Edward Jenner found that if experimental subjects were deliberately infected with cowpox, which caused only a mild illness, they are immune from smallpox. (A) which caused only a mild illness, they are immune from (B) causing only a mild illness, they become immune from (C) which causes only a mild illness, they are immune to (D) causing only a mild illness, they became immune from (E) which caused only a mild illness, they would become immune to https://www.nytimes.com/1979/02/04/archives/smallpox-is-not-dead.html The research at the Atlanta lab had its beginnings in 1796 when an English country physician named Edward Jenner found that if a person was deliberately infected with cowpox, which caused only a mild illness, he would be immune from smallpox. Although the practice of vaccinating people with cowpox spread quickly to other countries, smallpox maintained its hold on most of the world until the 20th century. Even after immunization against it became widespread in the industrialized nations, the majority of third‐world nations continued to suffer from it, at least until 1967, when an intensified international campaign was launched. Jenner and Cowpox (A) Verb ( are ) (B) Modifier / Meaning ( causing ); Verb ( become ); Idiom ( become immune from ) (C) Verb ( are ) (D) Modifier / Meaning ( causing ); Verb (became); Idiom ( became immune from ) (E) CORRECT First glance The beginning of each answer is either comma-which or comma-causing , indicating a modifier issue. Does this modifier refer just to the main noun before it or to the entire preceding clause? Issues (1) Modifier / Meaning: causing In this problem, the issue that you spotted during the first glance might be a little tricky. Decide whether you want to start there or whether you want to start with anything else that you may have noticed as you read the original sentence. Does the modifier ( which caused only a mild illness ) refer just to the main noun before it, cowpox? Or does the modifier ( causing only a mild illness ) refer to the entire preceding clause ( if subjects were deliberately infected )? If the modifier were referring to the entire previous clause, then the sentence would be saying that deliberately infecting someone causes only a mild illness , implying that the disease would be more severe if people caught it naturally. This is illogical; either way, the person still has the same disease. The modifier must be referring just to the noun cowpox . Eliminate choices (B) and (D) for an illogical meaning. (2) Verb: are; become; became The non-underlined portion of the sentence sets up a conditional structure: If subjects were deliberately infected …then what? For the second half, answers (A), (B), and (C) use a present-tense verb ( are or become ). Were is in past tense, however, so present tense doesn't match. Eliminate choices (A), (B), and (C). Choice (D) does use the simple past tense became , but even simple past is not good enough here. The were verb in the conditional structure indicates that the sentence is discussing an event that may or may not happen. This second action is conditional on the first action having already happened: If someone were deliberately infected, then that person would become immune. Eliminate (D) for a faulty verb tense match as well. (3) Idiom: become / became immune from The choices switch between immune from and immune to . Which is correct? In general, immune to is acceptable to use. It is sometimes possible to use immune from , but become immune from is not acceptable. When using the word become , say become immune to . Eliminate choices (B) and (D) for a faulty idiom. Note that answer (A) uses immune from but does not contain a form of the word become . This can arguably be acceptable; the OG solution does not call this out as incorrect, so assume that it is okay for the purposes of the GMAT. The Correct Answer Correct answer (E) uses the noun modifier structure which caused to refer to the noun cowpox . This choice also uses the correct verb tense to finish the conditional structure: If someone were infected with cowpox , then that person would become immune to smallpox .

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The English physician Edward Jenner found that if experimental subjects were deliberately infected with cowpox, which caused only a mild illness, they are immune from smallpox.

(A) which caused only a mild illness, they are immune from
(B) causing only a mild illness, they become immune from
(C) which causes only a mild illness, they are immune to
(D) causing only a mild illness, they became immune from
(E) which caused only a mild illness, they would become immune to

Ans:
Here we can eliminate 2 options as we have a modifier error here. "which" Vs "causing", Which is correctly modifying cowpox. "causing" will modify the whole clause "experimental subjects were deliberately infected" which would be wrong.
Come down to A, C and E
If past tense then would

Thus option E is correct

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