You are given a science question (hard-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.
--------
Question: A student plans to design a new toothpaste tube that will give the same amount of toothpaste every time it is squeezed. Which should the student do first when designing the new tube?  
 (A) make a data table of amounts of toothpaste (B) gather building materials (C) create models of old toothpaste tubes (D) draw a few possible solutions

Answer: D


Question: Tobias rode his bike on a road for a 2 hr period. On average, he passed a 1 km marker every 3 min during this period. Which of the following was his average speed for this 2 hr period?  
 (A) 10 km/hr (B) 15 km/hr (C) 20 km/hr (D) 25 km/hr

Answer: C


Question: A certain atom has 20 electrons, 21 neutrons, and 20 protons. What is the atomic mass of the atom?  
 (A) 20 (B) 21 (C) 41 (D) 61

Answer:
C