Identify $A$ to $D$ in the following diagram.

• A
  $A$-Filament,$B$-Pollen sac,$C$-Pollen grain,$D$-Line of dehiscence
• B
  $A$-Filament,$B$-Pollen sac,$C$-Line of dehiscence,$D$-Pollen grain
• C
  $A$-Filament,$B$-Line of dehiscence,$C$-Pollen sac,$D$-Pollen grains
• D
  $A$-Filament,$B$-Line of dehiscence,$C$-Pollen sac,$D$-Pollen grains