In an X-rays tube, electrons striking a target are brought to rest, causing X-rays to be emitted. In a paricular X-ray tube, the maximum frequency of the emitted continuum X-ray spectrum is `f_(o)`. If the votage across the tube is doubled, the maximum frequency is
A. `f_(o)//2`
B. `f_(o)//sqrt(2)`
C. `f_(o)`
D. `2f_(o)`