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 two protein molecules are essential for muscle contraction and relaxation?
(A) actin and myosin (B) collagen and tubulin (C) hemoglobin and myoglobin (D) amylase and polymerase
[EX A]: A

[EX Q]: When nitrogen-rich runoff from farms enters a nearby farm, it causes abundant growth of algal mat on the pond surface. As a result, underwater plants in the pond begin to die. Identify the limiting factor that is most responsible for causing the underwater plants to die.
(A) oxygen (B) sunlight (C) nitrogen (D) carbon dioxide
[EX A]: B

[EX Q]: A wetland was drained to build a mall. Two years later, there were no more toads in that area. Why did the toads disappear?
(A) The toads were destroyed by the construction equipment. (B) The toads died because toads cannot breathe out of water. (C) The toads were frightened and went into the woods. (D) The toads go their food from the wetland ecosystem.
[EX A]:
D