A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic about pandas and their cost in zoos. It does not need any other sentence for context. It is a general statement that sets the stage for the following information. Why other lines are not chosen? The other paragraphs are not chosen because they do not stand alone. For example, Paragraph 2 starts with 'Food alone...' which is not a complete idea by itself. Paragraph 3 starts with 'Giant pandas...' which needs more context to understand. Paragraph 4 starts with 'Bamboo, however...' which also needs previous information to make sense.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion about pandas and their food. It gives a final point about the challenges pandas face with their diet.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it continues the discussion about pandas. The phrase 'Food alone' shows that the topic is still about pandas and their care.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 because it explains why food is a headache. It gives more details about pandas' eating habits.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it explains the problems with bamboo as food. It shows the consequences of pandas eating bamboo.