The paragraph discusses the rise in peanut allergies and introduces the "hygiene hypothesis" as a potential explanation. Here's the breakdown:
1. Background: Incidence of peanut allergies doubled (1997-2002), prompting research into causes.
2. Hygiene Hypothesis (First Highlighted Sentence):
- Main idea: Immune system needs exposure to pathogens to strengthen itself.
- Overly sterile environments may lead the immune system to target harmless substances (e.g., peanuts).
3. Limitation (Second Highlighted Sentence):
- Even if the hypothesis is true, practical benefits are limited because people won't adopt unsanitary habits to prevent allergies.

Analyzing the Highlighted Sentences
First Highlighted Sentence:
- Introduces the "hygiene hypothesis" as a supported explanation for allergies.
- Role: Presents the primary theory (main idea) of the paragraph.

Second Highlighted Sentence:
- Notes the impracticality of acting on the hypothesis (people won't live less sanitarily).
- Role: Qualifies the hypothesis by pointing out its limitations (supporting the conclusion that the knowledge has mixed benefits).

Evaluating the Options
Option A:
- "First sentence states the main idea... second sentence states the secondary idea."
- Partially correct, but the second sentence doesn't introduce a new idea; it supports the conclusion about mixed benefits.
- Not the best fit.

Option B:
- "First sentence states a counterargument... second sentence provides a supporting detail."
- The first sentence is not a counterargument; it's the central theory. Incorrect.
- Reject.

Option C:
- "First sentence identifies a problem, second sentence argues a solution."
- The first sentence introduces a theory, not a problem. The second sentence doesn't propose a solution but a limitation.
- Reject.

Option D:
- "First sentence introduces the main idea... second sentence is a supporting statement for the conclusion."
- Correct.
- First sentence: Main idea (hygiene hypothesis).
- Second sentence: Supports the conclusion that the hypothesis's benefits are mixed (by explaining why people won't act on it).

Option E:
- "First sentence is the primary argument, second sentence is a secondary argument."
- The second sentence isn't an argument but a limitation/qualification.
- Less precise than D.

Why Option D is Best
- The first highlighted sentence introduces the core theory (hygiene hypothesis), which is the paragraph's main focus.
- The second highlighted sentence supports the concluding point (mixed benefits) by explaining practical limitations.

Why Others Are Incorrect
- A: Mislabels the second sentence's role.
- B, C, E: Mischaracterize the first sentence's role or the second sentence's purpose.

Final Answer
D. The first sentence introduces the main idea of the paragraph, and the second sentence is a supporting statement for the conclusion.

Correct Answer: D