Francis lets her infant sleep alone in a crib but makes sure that he is always on his back. Why might she do this?

a) Infants can breathe best when lying on their backs.
b)Infants grow most while lying on their backs, as their muscles can fully stretch out.
c)Infants can use their startle reflex only when lying on their backs.
d)Infants' cries cannot be heard unless lying on their backs.