The ionisation energies of K-shell for cobalt, copper, and molebdenum are 7.8, 9.0 and `20.1 KeV` respectively. If any metal out of these is used as target in an X-ray tube operated at `15KV`, then
(a) the K-series of characteristic X-rays will be emitted only by cobalt
(b) the K-series of characteristic X-rays will be emitted only by cobalt & Cu
(c) the K-series of characteristic X-rays will be emitted only by cobalt, Cu , Co and Mo
(d) the minimum wavelength of continuous X-rays emitted by the three metals will be same