Which of the following is correct with respect to the following statements? Due to diffusion of electrons from $n$ to $p$-side:
$I$. electrons are accumulated in the depletion region.
$II$. electron drift current is from $p$-side to $n$-side.
$III$. an ionised donor is left in the $n$-region.
$IV$. electrons of $n$-side come to $p$-side and electron-hole combination takes place in $p$-side.
Select the correct option from the following.

• A
  $I$ and $II$
• B
  $I$ and $III$
• C
  $I$ and $IV$
• D
  $II, III$ and $IV$