निम्नलिखित में से किसमें नियंत्रित श्रृंखला अभिक्रिया (नियंत्रित शृंखला अभिक्रिया) होती है?

Four statements regarding nuclear properties are given (where A is the mass number): A. The volume of a nucleus is proportional to A1/3. B. The volume of a nucleus is proportional to A. C. The difference in mass of an atom and its nucleus is called the mass defect. D. The difference in mass of a nucleus and its constituent nucleons is called the mass defect. Choose the correct answer from the options given below:

Which of the following is used as a moderator in a nuclear reactor?