A child swinging on a swing stands up. Then the time period of

a-child-swinging-on-a-swing-stands-up-then-the-time-period-of

A child swinging on a swing stands up. Then the time period of the swing will ___________

A. Increase

B. Decrease

C. Remain the same

D. Increase, if the child is long and decreases if the child is short

I got this question in a national level competition.

My doubt stems from Harmonic Motion topic in portion Oscillations of Engineering Physics I

Right option is B. Decrease

For explanation I would say: The child and the swing together constitute a pendulum of a time period,

T∝2π√(l/g)

As the girl stands up, her centre of gravity is raised. The distance between the point of suspension and the centre of gravity decreases, that is the length l decreases. Hence the time period T decreases.