When a beam of accelerated electrons hits a target , a continuous `X` - ray spectrum is emitted from the target. Which of the following wavelength is absent in `X` - ray spectrum , if the `X` - ray tube is operating at `40,000 "volts"`?
A. 0.25 Å
B. 0.5 Å
C. 1.5 Å
D. 1.0 Å
