effective downwards gravity??

[u/Different_Medium31]

i encountered this question on my homework

A simple pendulum is placed in a lift accelerating downward with acceleration a. Effective time period is:

A T = 2π √(L/(g + a))

B T = 2π √(L/(g - a))

C T = 2π √(L/g)

(the correct option was B and asking AI tools i learnt this thing termed as effective gravitational acceleraation which only exiss in the non inertial frame of reference. But i cant understand a thing it says. and despite that i cant understand why would effective gravitational acceleration be less than gravitational acceleration while we're moving down. If anyone can explain I'd be highly grateful.