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A simple pendulum is suspended from the roof of a trolley which moves in a horizontal direction with an acceleration a, then the time period is given by $\mathrm{T}=2\mathrm{\pi }\sqrt{\frac{\mathrm{l}}{\mathrm{g}\text{'}}}$,  where  is equal to

(a) g

(b) g-a

(c) g+a

(d) $\sqrt{{\mathrm{g}}^{2}+{\mathrm{a}}^{2}}$