What must happen before a chemical reaction can begin?
a.) the activation energy must be exceeded.
b.) the activation energy must be reached.
c.) the concentrations of products and reactants must be equal.
d.) the concentration of reactant molecules must be reduced.

The answer is B. A chemical reaction consists of an endothermic stage (energy taken in to break chemical bonds) followed by an exothermic stage (energy released as bonds are formed). The first stage requires this energy input, which is the activation energy. When the activation energy is reached or exceeded, the reaction will occur. Therefore as soon as this threshold is REACHED, it happens

I’m not exactly sure what your trying to ask, but if it’s how an organelle is similar to a body system or a part a nucleus is like a in function. but i don’t fully understand what you’re trying to ask