Why does burnt gases with content of N2, CO2, H2, O2 and H2O re-enter the boiler in overfeed system?

For complete removal of moisture

To dismiss the ash content in the gases formed

Why does burnt gases with content of N2, CO2, H2, O2 and H2O re-enter

1.	Why does burnt gases with content of N₂, CO₂, H₂, O₂ and H₂O re-enter the boiler in overfeed system?
A.	To dismiss the ash content in the gases formed
B.	Due to incomplete burning
C.	For complete removal of moisture
D.	To complete the burn the coal
Answer» C. For complete removal of moisture