Mendel's work remained unrecognized for a long time due to:
$I.$ Communication was not easy.
$II.$ The concept of factors which did not blend was not accepted.
$III.$ The use of mathematics to explain biological problems was unacceptable.
$IV.$ He could not provide any physical proof for the existence of factors.
Choose the right combination.

• A
  $I$ and $II$
• B
  $II$ and $III$
• C
  $III$ and $IV$
• D
  All of these