Robert knows that healthy, well-watered plants are quiet. He also knows that many insects (昆虫) prefer attacking dry plants rather than healthy plants. How do the insects know which are healthy plants and which are not? Robert thinks that the insects may hear the plants crying and then they may come to kill. . 56. There are many "water pipes"_________.

A. in a plant's stem

B. in a plant's peel(果皮)

C. in a plant's bark(树皮)

D. in the earth