What value was there in predicting the properties for gaps in mendeleev's table?

Predicting the properties for gaps in Mendeleev’s table helps discover new elements. Because during his presentation of his proposed periodic table, he noted some gaps in his table and predicted that there are still unknown elements between those gaps. He uses some prefixes like dvi-, tri-, and eka- in order to give provisional names to his predicted elements. In his periodic table, he also laid out his elements from the lightest to the heaviest element. The lightest elements started from zero, some examples are gases.

The answer to the question: choose the correct statement.  a) when ethane reacts with fluorine gas the product is less stable.b) addition reactions of unsaturated hydrocarbons and halogens requires a high activation energy.  c)the double covalent bond within unsaturated hydrocarbons requires a lot of energy to break.  d)the double covalent bond within unsaturated hydrocarbons can be broken easily.is: a) when ethane reacts with fluorine gas the product is less stable.

Ithink the answer is 10.576