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.

d. The introduction of the Coolidge tube allowed for an x-ray output that could be predetermined and accurately controlled.