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Correct Answer: Option (A) — be shorter
Concept & Option Breakdown
Standard Reference VerifiedA swing acts as a simple pendulum whose time period is given by $T = 2\pi \sqrt{\frac{L}{g}}$, where $L$ is the effective length from the pivot point to the centre of mass of the system. When the girl stands up, her centre of mass shifts upwards, decreasing the effective length $L$. Since $T \propto \sqrt{L}$, a decrease in $L$ causes the time period $T$ to become shorter.
Standing raises the centre of mass $\rightarrow$ reduces effective length $L$ $\rightarrow$ decreases time period $T$.
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