$A.$ Fucus algae exhibits a diplontic life cycle.
$R.$ In Spirogyra algae,the diploid zygote is the dominant and active stage.

• A
  $A$ and $R$ are both true,and $R$ is the correct explanation of $A$.
• B
  $A$ and $R$ are both true,but $R$ is not the correct explanation of $A$.
• C
  $A$ is true and $R$ is false.
• D
  $A$ is false and $R$ is true.