Final answer:
Flat mirrors can focus sunlight at the top of a tower because the Sun is so far away that its rays are nearly parallel when they reach Earth, which is crucial for concentrating those rays efficiently. So, the best answer is b, the Sun is essentially infinitely far away, so its rays are parallel when they strike the mirrors.
Step-by-step explanation:
Using flat mirrors to focus the Sun's rays at the top of a tower, as is done by the power plant shown in the figure, is only possible because the Sun is essentially infinitely far away, so its rays are parallel when they strike the mirrors.
This parallel nature of sunlight is due to the great distance between the Sun and the Earth.
At such a distance, light rays emanating from the Sun arrive at Earth nearly parallel to each other.
This characteristic is crucial for the optimal functioning of solar power plants using flat mirrors because they rely on the parallelism of sunlight to concentrate it efficiently at a single point, such as the top of a tower.
While adjustable mirrors controlled by computers (option D) do help in continuously focusing the light as the Sun moves across the sky, the fundamental principle that allows this system to work is the near-parallel nature of sunlight, so the correct answer is B.
So, the best answer is b, the Sun is essentially infinitely far away, so its rays are parallel when they strike the mirrors.