Why Does an Embryo Undergo the Process of Gastrulation

Why does an embryo undergo the process of gastrulation?

A) Gastrulation separates the cells of the inner cell mass and the trophoblast so that the trophoblast is free to develop into the placenta.
B) During gastrulation,the cells of the embryo move around to create the four extraembryonic membranes that protect the fetus.
C) The rapid cell division that occurs during gastrulation allows the embryo to increase quickly in cell number without using much energy for cell growth.
D) Cells in the blastocyst differentiate to form embryonic tissues but are not originally located in the right place.Gastrulation moves these cells to the proper position in the embryo.