Organisms which "hold an ecosystem together" because they have a significant role to play are called A. endemic species
B. native species
C. niche species
D. keystone species

Which description provides the best objective summary of the article in the passage?

Which of the following best describes the purpose of the parathyroid hormone? a. it decreases the blood calcium level and decreases blood phosphorus level. b. it decreases the blood calcium level and increases the blood phosphorus level. c. it increases the blood calcium level and increases blood phosphorus level. d. it increases the blood calcium level and decreases blood phosphorus level.

Fill in the blank with the best word to complete the sentence. white blood cells protect against and aid in blood clotting when injuries are sustained.