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?
Explanation
Context: Heliographs were effective over long distances but only during daytime and were dependent on air opacity and line of sight — environmental factors. The blank states what kind of technology eventually replaced them.
Answer: B) was not so constrained by environmental circumstances.
Why? The passage identifies heliographs' key limitations as environmental (daylight, air clarity, line of sight). The replacing technology overcame these constraints — B directly states this characteristic.
Traps:
- A) worked on similar principles but was easier to produce → about production cost, not the environmental constraints that were the actual problem
- C) could be used for more than military, surveying, or forestry purposes → about application range, not the core limitations
- D) didn't require knowledge of Morse code → about user skill requirements, not the environmental limitations