The table shows the numbers of particles in a nucleus before and after one decay.
| Protons | Neutrons | |
|---|---|---|
| Before | 15 | 15 |
| After | 14 | 16 |
Which particle is emitted, and what happens to the mass number?
An electron is emitted; the mass number decreases by one.
A positron is emitted; the mass number decreases by one.
A positron is emitted; the mass number is unchanged.
An electron is emitted; the mass number is unchanged.