In natural circulation type boiler _______________?

A. heating takes place at bottom and the water supplied at bottom gets converted into the mixture of steam bubbles and hot water which rise to drum
B. water is supplied in durm and through down-comers located in atmospheric conditon it passes to the water wall and rises to drum in the form of mixture of water and steam
C. feed pump is employed to supplement natural circulation in water wall type furnace
D. water is converted into steam in one . pass without any recirculation
E. water is fed at atmospheric pressure