What lead mendeleev to believe there were 2 missing elements between 62.5 and 75

im not exactly sure but i think its b!

the correct answer is:

a. formation of precipitation

a. energy is absorbed in the reaction.

if in a given reaction the energy of products is greater than the energy of reactants it means that reaction is endothermic and during the reaction, the energy is absorbed by reactants. in an endothermic reaction, the products have more stored chemical energy than the reactants. and in this reaction the reactants are more stable than products before the compounds with less energy are always more stable.