53. The "immunotolerance" of a pregnant woman toward her unborn child is the result of
(A) the complete physical separation from her cells and those of the unborn child.
(B) the tenacity with which the unborn child's immune system counteracts the woman's immune system.
(C) the unborn child having enough of the woman's identity so as to escape detection as foreign.
(D) modern medical intervention during every pregnancy.
(E) the relative quiescence of a pregnant woman's immune system compared to when she was not pregnant.