NASA uses a launch clock to measure time before and after lift-off. It takes 2 hours and 15 minutes of preparations on base, 20 minutes to travel to the launch pad, and 40 minutes of pre-launch checks once in the spacecraft. When should the astronauts begin preparation? –3 hours 15 minutes, because all the preparation times add to 3 hours and 15 minutes
–3 hours 35 minutes, because all the preparation times add to 3 hours and 35 minutes
–2 hours 35 minutes, because all the preparation times add to 2 hours and 35 minutes
–2 hours 15 minutes, because all the preparation times add to 2 hours and 15 minutes