You are given a science question (easy-level) and four answer options (associated with "A", "B", "C", "D"). Your task is to find the correct answer based on scientific facts, knowledge, and reasoning. Do not generate anything else apart from one of the following characters: 'A', 'B, 'C', 'D'. There is only one correct answer for each question.

[EX Q]: Which statement correctly describes a relationship between two human body systems?  
 (A) The digestive system absorbs nutrients which are transported by the nervous system. (B) The digestive system absorbs nutrients which are transported by the circulatory system. (C) The circulatory system exchanges gases which are transported by the nervous system. (D) The circulatory system exchanges gases which are transported by the digestive system.
[EX A]: B

[EX Q]: Florida has a large supply of phosphate that ancient seas deposited millions of years ago. The phosphate contains the remains of animals that were deposited in layers on the sea floor. Which type of rock did the phosphate become?  
 (A) intrusive igneous (B) extrusive igneous (C) clastic sedimentary (D) organic sedimentary
[EX A]: D

[EX Q]: Students on two different school campuses are comparing the growth rate of grass three weeks after fertilizer has been applied. The same fertilizer and the same amount of water are used on both campuses. Which additional variable is most important to control when the results of the two investigations are compared?  
 (A) type of grass used (B) amount of trees in the area (C) the weather conditions of the day (D) the time of day the measurements are taken
[EX A]:
A