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.

In which case does a physical change occur?  
 (A) Ice melts into water. (B) Bread burns in a toaster. (C) A cake bakes in an oven. (D) Two chemicals form a new substance.
A

Why are some rocks smoother and rounder than others?  
 (A) They have the shapes of the minerals from which they are made. (B) They were smoothed by the action of wind and water. (C) They were squeezed up through the soil. (D) They hardened in rounded pockets of soil.
B

The charge of the nucleus of an atom is due to the presence of  
 (A) positive protons. (B) negative neutrons. (C) negative protons and positive electrons. (D) positive protons and negative neutrons.

A