Useful Thermal Expansion in Practical Applications

Steps:

• Define useful thermal expansion as expansion intentionally harnessed for a beneficial purpose, like measurement or easing access.
• Evaluate P: Liquid expansion in a thermometer directly measures temperature changes, serving a clear function.
• Evaluate Q: Lid expansion on a glass container (metal expands more than glass) loosens the fit, making it easier to open when heated.
• Evaluate R: Beam expansion in roof supports risks buckling or misalignment, creating structural issues rather than benefits.

Why B is correct:

• B identifies P and Q as useful per the principle that thermal expansion is advantageous when designed into devices (e.g., thermometers) or processes (e.g., differential expansion for loosening), excluding problematic cases like R.

Why the others are wrong:

• A includes R, which demonstrates harmful expansion, not useful application.
• C omits Q, ignoring its practical benefit in container access.
• D includes R (problematic) and excludes P (essential for measurement).

Final answer: B