Which is the correct diagram of a half-wave rectifier?

In a half-wave rectifier using a $PN$ junction diode,the input voltage has an amplitude of $25 \ V$ and a frequency of $50 \ Hz$. No filter is used,and the load resistance is $1000 \ \Omega$. The forward resistance of the diode is $10 \ \Omega$. The $d.c.$ power output and $a.c.$ power input are respectively:

The maximum efficiency of a full-wave rectifier is .........$\%$

Why is a junction diode used for rectification?

The output sinusoidal current versus time curve of a rectifier is shown in the figure. The average value of output current in this case is

The output current versus time curve of a rectifier is shown in the figure. The average value of the output current in this case is