Instructions: 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.
Input: In a laboratory activity, four different types of rock were placed in identical acid solutions for one week. Each sample was then measured to see if any of the rock mass was dissolved by the acid solution. Which is the dependent (responding) variable in this activity?
(A) the mass of each rock (B) the type of rock tested (C) the length of time of the test (D) the amount of dissolved rock
Output:
D