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.
Q: Some students measure the mass of a small pile of leaves before and after the leaves decompose. Each time the students measure the mass, they place a container with the decomposing leaves on a balance. Before recording the mass of the leaves, the students must do which of the following?
(A) Subtract the mass of the container (B) Add leaves from other plants to the container (C) Allow the leaves to settle in the container for a few days (D) Use a metric ruler to measure the volume of the container
A:
A