Decrease in external air pressure causes gas expansion

Steps:

• Aircraft ascends to higher altitude where atmospheric pressure decreases.
• Sealed packet traps fixed amount of air inside at constant internal pressure initially.
• Lower external pressure allows internal air to expand per Boyle's law.
• Expansion increases packet volume, making it larger.

Why C is correct:

• Boyle's law (P1V1 = P2V2) states that for fixed gas amount, volume increases as external pressure decreases.

Why the others are wrong:

• A: Density decreases as volume expands while mass stays constant.
• B: Mass of air and packet remains unchanged in sealed system.
• D: Volume increases due to expansion, not decreases.

Final answer: C