The shedding of male gametophyte in Selaginella occurs at 13-celled stage which consists of
A. 8 jacket cells, 1 generative cell, and 4 androgonial cells
B. 9 jacket cells and 4 androgonial cells
C. 12 jacket cells and 1 male gamete
D. 8 jacket cells, 1 prothalial cell, and 4 androgonial cells