A heliograph is a semaphore device used for sending optical communications--usually in the form of Morse code--by reflecting flashes of sunlight off a mirror. Heliographs were used for rapid communication across expansive distances for military, surveying, and forestry purposes during the late nineteenth and early twentieth centuries, but they were largely effective only during the daytime, and the range of the device depended on factors such as the opacity of the air and line of sight. Therefore, heliographs were eventually replaced by technology that _______

Which choice most logically completes the text?

Question

Accepted Answer

was not so constrained by environmental circumstances.

Answer

worked on similar principles but was easier to produce and maintain.

Answer

could be used for more than military, surveying, or forestry purposes.

Answer

enabled communication that didn't require knowledge of Morse code.