A certain nucleus P has a mass number 15 and atomic number 7. The number of neutrons is :

1. 7
2. 15
3. 8
4. none of the above

The atoms of the same element, having the same atomic number Z but different mass number A are called :

1. Isotopes
2. Isobars
3. Isotones
4. None of the above

³⁹₁₉K and ⁴⁰₂₀Ca are :

1. Isotopes
2. Isobars
3. Radioactive substances
4. Isotones

Out of the following, radioactive substances are :

1. Uranium
2. Polonium
3. Actinium
4. All of the above