First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of the text. It gives a clear idea about what the text will discuss. This follows the rule of Topic Sentence. Why other lines are not chosen? Other paragraphs are not chosen because they do not stand alone or do not introduce the main topic. For example, 'Two types of bonds exist in materials, Wang explains.' starts with a reference to Wang, which needs prior context. 'There are covalent bonds...' uses 'there are' which does not introduce a new idea. 'For example...' also needs a previous sentence to understand what it is explaining.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it provides an example that wraps up the discussion about chemical bonding. It gives a clear illustration of non-covalent bonds, which relates back to the main topic. This follows the Summary Style rule.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it gives more information about the topic introduced in Paragraph 1. This follows the Logical Order rule, as it continues the discussion about chemical bonding.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 because it explains the two types of bonds mentioned in Paragraph 2. This follows the Broader to Specific Strategy, as it gives specific details about the types of bonds.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it gives an example of non-covalent bonds. This follows the Topic Example rule, as it provides a specific case of the general information given in Paragraph 3.