TOEFL Reading: ETS-TOEFL阅读机经 - 15P5239SUW3N7AGFL$

In paragraph 4, why does the author mention that tree frog calling is said to be "the most energetically expensive behavior yet measured in any vertebrate?" A. explain why it is important for tree frogs to be in top physical condition B. To distinguish tree frogs from other species of frogs C. To indicate that survival is more difficult for frogs than for other vertebrates D. To emphasize how physical effort calling requires