What does the $K\alpha$ characteristic $X$-ray represent?

The characteristic $X-$ray radiation is emitted,when

$20 \ kV$ electrons can produce $X$-rays with a minimum wavelength of

The typical wavelength of $X$-ray is

$\Delta \lambda$ is the difference between the wavelength of the $K_\alpha$ line and the minimum wavelength of the continuous $X$-ray spectrum when the $X$-ray tube is operated at a voltage $V$. If the operating voltage is changed to $V / 3$,then the above difference is $\Delta \lambda^{\prime}$. Then:

Absorption of $X$-rays is maximum in which of the following different sheets?