First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of the text. It talks about psychologists and their focus on validity and reliability. This is a broad overview of the subject. Why other lines are not chosen? The other paragraphs do not work as the first sentence. For example, "Validity is defined as the same" starts with a specific idea and does not introduce the topic. "For example, when a survey is asking about someone’s personality, it shouldn’t ask him chemistry questions" starts with 'For example,' which is a conjunction. "Meanwhile, a survey also values reliability" also starts with a conjunction. So, they cannot be the first sentence.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion about surveys. It adds a final point about reliability, which is important to the overall theme of validity and reliability.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it explains what validity means. This is a logical order where the general statement about psychologists leads to a specific definition.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 because it gives an example of validity. The example shows how validity works in surveys, making it a clear flow from definition to illustration.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it introduces another important concept: reliability. The word 'Meanwhile' shows that this paragraph adds more information about surveys, linking the ideas.