Study the four statements $(A-D)$ given below and select the two correct ones out of them.
$A.$ Definition of biological species was given by Ernst Mayr.
$B.$ Photoperiod does not affect reproduction in plants.
$C.$ Binomial nomenclature system was given by $R$.$H$. Whittaker.
$D.$ In unicellular organisms,reproduction is synonymous with growth.
The two correct statements are:

• A
  $B$ and $C$
• B
  $C$ and $D$
• C
  $A$ and $D$
• D
  $A$ and $B$