Answer: controller.
Justification:
A fission reaction is the split of the nucleus of an atom due to the bombardment with neutrons.
A fission reaction may terminate in a reaction chain, a controlled reaction or extinguish.
When for every 2 or 3 neutrons released, only one neutron is allowed to strike a fissionable nucleus, the reaction is controlled.
When the the ratio is less than one then the reaction will be extinguished.
When the number of reactions grow, this is a reaction chain, where the reaction is out of control, and the consequence is the release of vast amount of energy (heat) is a short time, causing a nuclear explosion.
Phase transitions is when a substance go through a change in its physical state, for example, solid to liquid, liquid to gas, etc. Energy in this case must be absorbed or released, and not necessarily it will be seen a change of temperature in this case. Mathematically, we calculate the energy released or absorbed by using the molar enthalpy of vaporization or molar enthalpy of fusion, as we can see in the following formulas:
ΔH = m*ΔHvap
ΔH = m*ΔHfusion
So phase transition is the energy required for a substance to change the physical state.
I'm guessing it will be B its the only one that fits with that sentence
Half life is 15 hours
so 30 hours would be 2 half lives
after 15 hours it would be at 50%
after 15 more hours it would be at 50% of 50%
so .5 x .5 = .25
it would be 25%