What is a theory to explain why so many women experience morning sickness during pregnancy?

What is a theory to explain why so many women experience morning sickness during pregnancy?
 
  a. Because of the nausea associated with morning sickness, most women will avoid traveling, which thus reduces the risk of harming of the embryo.
  b. Morning sickness often forces women to rest more and take it easy, reserving more nourishment for the embryo.
  c. Morning sickness sends a strong signal to others that pregnant women need to be taken care of.
  d. Morning sickness may cause women to expel and avoid substances that may otherwise be toxic to the developing embryo.