When is it safe to launch a high power rocket?

For a safe high power rocket launch, ensuring that all necessary precautions have been taken is essential. The correct answer emphasizes the importance of both moving spectators to safe distances from the launch pads and ensuring that the sky is clear. This is crucial to prevent accidents related to the rocket's flight path and to protect spectators from potential hazards.

Making sure that all persons have moved to safe distances ensures that they are not in harm's way in case the rocket experiences an anomaly during launch. Additionally, having a clear sky allows for better visibility and helps to avoid any unforeseen complications that might arise from weather conditions, such as strong winds or lightning.

The requirement of warning spectators and giving a countdown is also a critical safety measure. This gives everyone an opportunity to prepare for the launch, ensuring that they are focused and aware of what is happening.

Combining these two factors—maintaining a safe distance and confirming clarity in the environment—creates an additional layer of safety. Therefore, it is important to fulfill both conditions before proceeding with the launch, justifying why the correct choice encompasses both A and B.