The most significant advancement in the early days of radiography came in 1913 when W. D. Coolidge ...

The most significant advancement in the early days of radiography came in 1913 when W. D. Coolidge
 
  A) became the first to alert the profession of the dangers of radiation exposure and advocated for the science of radiation protection.
  B) exposed a prototype of a dental radiograph, setting the stage for preventive oral health care.
  C) developed a variable, high kilovoltage x-ray machine compatible with the paralleling technique.
  D) introduced an x-ray tube that allowed for an x-ray output that could be predetermined and accurately controlled.