$S -$ Statement: In the pachytene substage,each group of chromosomes appears as a bivalent.
$R -$ Reason: During prophase in plant cells,the centriole units arranged at the two poles form spindle fibers.

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