OE
Look for differences across the answer options to identify what is being tested in the sentence.
The options are differing in the construction used to suggest that a study is needed. Some options are saying “it has become imperative to study”, others are saying “studying has become imperative” and (C) removes the word “imperative” and simply says “has to be studied”
What has to be studied is also different in the sentences. Should we study the effects, the plumes or how plumes affect the ozone?
Finally, the options are different in how the underlined portion begins.
Step 2 of solving this GMAT Sentence Correction Question: Understanding the Sentence
Because many more rocket launches are likely in the future, the effects of the exhaust on the stratosphere have to be studied.
The effects of the plume on the stratosphere have to be studied. An option that suggests that the plume has to be studied would be incorrect. It might well be impossible to study the actual plume because to do so will require that it be contained and not dispersed in the atmosphere.
Step 3 of solving this GMAT Sentence Correction Question: Eliminating Options
Option (C) suggests that the plume has to be studied, when in fact, the effects on the stratosphere have to be studied.
Option (D) also suggests that the plume has to be studied – again, impossible. Option (D) also uses the singular verb “has” to refer to the two studies – of the plume and of the effect.
Option (E) does not have subject-verb agreement either. The option uses the singular verb “affects” to refer to the plural “plumes”. Moreover, both options (E) and (A) use the construction “it has become imperative”. In this construction, the pronoun “it” does not have a noun to reference and such a generic usage of the same is considered ambiguous.
Choice B is the correct answer.