3. A journalist reports that they are leaving Las Vegas tomorrow night
It
They
4. The lecturer thought that Columbus never realized that he had discovered America.

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3. A journalist reports that they is leaving Las Vegas tomorrow night.
Explanation: In this sentence, we are referring to a journalist, which is a singular noun. Therefore, we should use the pronoun "he/she/it" to agree with the singular noun. In this case, "he/she/it" is referring to the journalist, so the correct pronoun to use is "he/she/it." Since the journalist is not specified as either male or female, we can use the neutral pronoun "they" as well. However, it is more common to use "he/she" for a singular person.

Step-by-step solution:
1. Identify the subject of the sentence: the journalist.
2. Determine the appropriate pronoun: "he/she/it" or "they."
3. Since the journalist is a singular noun, we can use "he/she/it" or "they" as the pronoun in this sentence.
4. Choose the appropriate pronoun based on the gender or use the neutral pronoun "they."
5. In this case, we can use "he/she" or "they" as the pronoun for the journalist: "he is leaving Las Vegas tomorrow night" or "they are leaving Las Vegas tomorrow night."

4. The lecturer thought that Columbus never realized that he had discovered America.
Explanation: In this sentence, we are referring to Columbus, who is a male historical figure. To maintain consistency in the pronouns used, we should use the pronoun "he" to refer to Columbus.

Step-by-step solution:
1. Identify the subject of the sentence: Columbus.
2. Determine the appropriate pronoun: "he/she/it" or "they."
3. Since Columbus is a male historical figure, we can use the pronoun "he" to refer to him.
4. In this sentence, the lecturer thought that Columbus never realized that he had discovered America, where "he" refers to Columbus's realization.
5. Therefore, the corrected sentence would be: "The lecturer thought that Columbus never realized that he had discovered America."