| (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. | 
| 1. | (a) and (c) | 2. | (a) and (d) | 
| 3. | (b) and (d) | 4. | (a) and (b) | 
| 1. | The conservation of charge | 
| 2. | The conservation of mass | 
| 3. | The conservation of mass energy | 
| 4. | The conservation of momentum |