An X-ray tube is operating at 50kV and 20mA. The target material of the tube has mass of 1kg and specific heat 495  ${\mathrm{Jkg}}^{-1}{}^0{\mathrm{C}}^{-1}$ . One percent of applied electric power is converted into  X-rays and the remaining energy goes into heating the target. Then,

a suitable target material must have high melting temperature

the average rate of rise of temperature of the target would be  $2^{°}{\text{Cs}}^{-1}$

a suitable target material must have low thermal conductivity

the minimum wavelength of X-rays emitted is about  $0.25\times {10}^{-10} \text{m}$