In a nuclear reactor, moderators slow down the neutrons which come out in a fission process. The moderator used have light nuclei. The heavy nuclei will not serve the purpose, because:
| 1. | they will break up |
| 2. | elastic collision of neutrons with heavy nuclei will not slow them down |
| 3. | the net weight of the reactor would be unbearably high |
| 4. | substances with heavy nuclei do not occur in the liquid or gaseous state at room temperature |
| (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) |