The diagram below shows some subatomic particles. A diagram similar to a flowchart starts with a circle labeled Atom. From there, a line leads to a small circle labeled Electron and a larger one labeled Nucleus. From nucleus, two lines lead to a group of circles, labeled Neutrons and protons. From there, two lines lead to a circle surrounding two blue and one orange smaller circles.
What is the particle that is labeled with a question mark (?) in the diagram?
A. beta particle
B. quark
C. isotope
D. alpha particle