$A-$ The $C_3$ and $C_4$ plants respond differently to $CO_2$ concentrations.
$R -$ The concentration of $CO_2$ in the atmosphere is between $0.03$ and $0.04$ percent. An increase in concentration up to $0.05$ percent can cause an increase in $CO_2$ fixation rates.

• A
  $A$ and $R$ both are correct.
• B
  $A$ and $R$ both are incorrect.
• C
  $A$ is correct and $R$ is incorrect.
• D
  $A$ is incorrect and $R$ is correct.