The word "tenuous" (4th paragraph) most probably mean ______.

A．John Donne contributed to medical development through the medical
B．There has been no fantastic spurt in medical development until the present century
C．Medical development has led to the rise in infant mortality
D．Medical development will bring us a bright future

A．other animals have more control over themselves
B．humans need to cut out certain habits to achieve a healthy state
C．the different foods they eat take a hand in their different health condition
D．man's health depends on the medical treatment he gets