Which safety feature works to slow down nuclear-fission chain reactions?

answer; i believe the correct answer is (solid gold);

i just calculated it all out, and not including weekends, and all the different days off. as of this school year (2018-2019) students are spending 2198 hours in school this year

b. accuracy of the data matters alot more than its number of digits, organization or amount of detail. quantitative observation is more accepted than the qualitative observation.