Here's a problem to solve: Two forms of boron are naturally occurring, boron-10 and boron-11. What difference exists between a boron atom with a 10 atomic mass units (amu) and a boron atom with 11 atomic mass units (amu)?

Among the 4 following options, which is the correct answer?

- A: amount of neutrons
 
- B: number of electrons
 
- C: size of protons and neutrons
 
- D: size of electrons and electron cloud
 