The partial twinning that led to the births of conjoined twins Abby and Brittany Hensel must have happened during the first two weeks of gestation, because the girls
A. have two heads.
B. share tissues that descend from ectoderm, endoderm, and mesoderm.
C. share only tissues derived from mesoderm.
D. were born on time.
E. have two separate nervous systems.
B