GMAT Reading Comprehension (RC) | Re: Ultimate pendulum Clock - William Shortt

My reasoning for the answers:

1. Inference
P1: The ultimate pendulum clock, indeed theultimate mechanical clock of any kind , was invented by a British engineer, William Shortt. The first was installed in the Royal Observatory in Edinburgh in1921 .
P1: Every 30 seconds the secondary pendulum sentan electrical signal to give a nudge to the primary pendulum.
Inference flip: A. Before 1921 no one had designed a clock that used electricity to aid in its timekeeping functions.

2. According to passage:
P1:
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