Passage One Atoms, the basic building blocks of matter, are made of three basic components: protons, neutrons and electrons. The protons and neutrons cluster together to form the nucleus, the central part of the atom, and the electrons orbit about the nucleus. Protons and electrons both carry an electrical charge(电荷). The charges they carry are opposite to each other; protons carry a positive electrical charge while electrons carry a negative electrical charge. Neutrons carry no charge at all. Electricity is the movement of charged particles, usually electrons, from one place to another. Materials that electricity can move through easily are called conductors. Most metals, such as iron, copper and aluminum, are good conductors of electricity. Other materials, such as rubber, wood and glass, block the flow of electricity. Materials which prevent the flow of electricity are called insulators (绝缘体). Electrical cords are usually made with both conductors and insulators. Electricity flows through a conductor in the center of the cord. A layer of insulation surrounds the conductor and prevents the electricity from "leaking" out. Objects usually have equal numbers of positive and negative charges, but it isn’t too hard to temporarily create an imbalance. One way scientists can create an imbalance is with a machine called a Van de Graaff generator. It creates a large static charge by placing electrons on a metal dome(钟形罩) using a motor and a big rubber band. Since like charges repel, the electrons push away from each other as they collect on the dome. Eventually, too many electrons are placed on the dome and they leap off, creating a spark that looks like a bolt of lightning. Have you ever received a shock after having walked across a carpet This shock was caused by extra electrons you collected while walking across the carpet. Your body became like the dome of the Van de Graaff generator, full of extra electrons looking for a way to get away. The path back to the carpet was blocked by the shoes you were wearing, but they were able to move through your hand and into the object that you touched, causing the shock. So, the next time you walk across a carpet and shock your friend on the ear, tell them you were just trying to be a Van de Graaff generator! Which of the following is true within an atom according to the passage

A. The charges of protons equal the charges of electrons.
B. The charges of neutrons equal the sum of the charges of electrons and protons.
C. The charges of protons equal the sum of the charges of electrons and neutrons.
D. The charges of electrons equal the sum of the charges of protons and neutrons.