Correct Answer: A

Rationale: The word 'operate' is the correct choice as it agrees with the verb tense used in the sentence. In this case, the sentence requires the simple past tense. The correct form should be 'Two students operate the machine while the third oversaw their work.' Choices B, C, and D are incorrect because they do not match the required verb tense for the sentence. 'Operating' (choice B) is a present participle, 'were operating' (choice C) is past continuous, and 'has been operating' (choice D) is present perfect continuous, none of which fit the context of the sentence.

Correct Answer: A

Rationale: In this sentence, the word 'flock' is singular, so the pronoun that follows should also be singular. Therefore, the correct choice is 'its,' which refers back to the singular noun 'flock.' Choice B ('their') is incorrect because it is a plural pronoun and does not agree in number with the singular noun 'flock.' Choices C ('it's') and D ('they're') are both incorrect contractions that do not fit grammatically in this context.

Correct Answer: C

Rationale: In this sentence, the pronoun should refer to Luis, who borrowed the money. Since Luis is a male, the correct pronoun to use in this context is 'him.' Therefore, the correct answer is 'him.' Choices A, B, and D are incorrect as they do not match the gender of Luis or do not make sense in the context of the sentence.

Correct Answer: B

Rationale: The word 'envelope' is used incorrectly in the sentence. The correct word should be 'folder,' as mentioned in the original sentence. An envelope is a flat paper container for a letter or documents, while a folder is a folded cover for papers or documents. 'Give,' 'whoever,' and 'replies' are all used appropriately in the sentence, so they are not the incorrect words.

Correct Answer: A

Rationale: The word 'work' (option A) is the most suitable choice for the sentence. The sentence is in the simple present tense and is discussing a general scenario or experience. The present simple tense is used to describe habits, routines, or general truths. Therefore, the word 'work' best fits in this context to indicate a general experience at County Medical without specifying a particular timeframe. Choices B, C, and D create incorrect tenses or imply specific past actions, which do not align with the intended meaning of the sentence.