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.

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
B

Which of the following is the best observation of the refraction of light?
(A) Looking at oneself in a mirror (B) Looking at a shadow of an object (C) Viewing an object through an open window (D) Viewing an object partially in water
D

Hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH) to produce a saltwater solution. What are the products of this chemical reaction?
(A) NaCl and H_{2}O (B) NaOH and HCl (C) HCl and NaCl (D) NaOH and H_{2}O
A