Epiboly is the process of

• A
  Rotation of gastrula within vitelline membrane so that animal pole becomes anterior
• B
  Overgrowth of micromeres which divide rapidly and spread downward over megameres except at yolk plug
• C
  Mass migration of cells from animal hemisphere so that upper micromeres migrate over edge of dorsal lip,roll inside and tucked beneath outer layer
• D
  Formation of small slit like invagination upon grey crescent