Which of the Following Is True of the Embryonic Stage

Which of the following is true of the embryonic stage of gestation?

A) Defects that occur in this stage are often the most serious and are likely to cause a spontaneous abortion.
B) The first bone cells appear during this stage.
C) The embryo becomes a fetus at the beginning of this stage.
D) Some cells around the edge of the blastocyst cluster to form a thickened cell mass from which the embryo begins to develop.