19. Celery stalks that are immersed in fresh water for several hours become stiff and hard. Similar stalks left ina salt solution become limp and soft. From this we can deduce that the cells of the celery stalks are
(A) hypotonic to both fresh water and the salt solution.
(B) hypertonic to both fresh water and the salt solution.
(C) hypertonic to fresh water but hypotonic to the salt solution.
(D) hypotonic to fresh water but hypertonic to the salt solution.