Fusion processes, like combining two deuterons to form a `He` nucleus are impossible at ordinary temperature and pressure. The reasons for this can be traced to the fact:

Options

A. nuclear forces have short range
B. nuclei are positively charged
C. the original nuclei must be completely ionized before fusion can take place
D. the original nuclei must first break up before combining with each other

Explanation

To understand why fusion processes, such as combining two deuterons to form a helium nucleus, are impossible at ordinary temperatures and pressures, we can break down the explanation into a series of steps:

### Step-by-Step Solution:

1. **Definition of Nuclear Fusion**:
- Nuclear fusion is the process where two small atomic nuclei combine to form a larger nucleus. In this case, we are considering two deuterons (which are isotopes of hydrogen) fusing to create a helium nucleus.

2. **Conditions for Fusion**:
- Fusion requires specific conditions, primarily high temperatures and pressures. This is because the nuclei involved are positively charged and repel each other due to the Coulomb force.

3. **Coulomb Repulsion**:
- When two positively charged nuclei (like deuterons) approach each other, they experience a repulsive force due to their positive charges. This is known as Coulomb repulsion. At ordinary temperatures and pressures, this repulsive force is significant.

4. **Nuclear Forces**:
- Once the nuclei are close enough, they experience a strong nuclear force, which is attractive and acts over a very short range. However, this nuclear force only becomes significant when the nuclei are very close together, typically at distances on the order of femtometers (10^-15 meters).

5. **Comparison of Forces**:
- At ordinary temperatures and pressures, the kinetic energy of the deuterons is not sufficient to overcome the Coulomb repulsion. The repulsive force between the two deuterons is greater than the attractive nuclear force, preventing them from getting close enough to fuse.

6. **High Temperature and Pressure**:
- At high temperatures, the kinetic energy of the deuterons increases, allowing them to overcome the Coulomb barrier. High pressure can also help bring the nuclei closer together, increasing the likelihood of fusion.

7. **Conclusion**:
- Therefore, fusion processes like combining two deuterons to form a helium nucleus cannot occur at ordinary temperatures and pressures due to the dominance of Coulomb repulsion over nuclear forces.