Highly reactive metals are found in combined state in nature. The natural substances containing metal in combined form are known as minerals. A mineral is called ore if:

Statement I: The third period contains only $8$ electrons and not $18$ like fourth period.

Statement II: In third period, filling starts from $3{\mathrm{s}}^1$ and complete at $3{\mathrm{p}}^6$ whereas in fourth period it starts from $4{\mathrm{s}}^1$ and complete after $3{\mathrm{d}}^{10}$ and $4{\mathrm{s}}^2$.

Consider the following statements.

I. In Mendeleef's Periodic Table, position of the isotopes was not fixed.

II. In the $19\mathrm{th}$ century, Mendeleef propounded periodic law is based on atomic mass.

Which of the above statement(s) is/are correct?

Consider the following substances.

I. Heavy water

II. Graphite

Which of the substances given above is/are used as a moderator in a nuclear reactor?

Statement I: Oxyhydrogen flame produces higher temperature than oxy atomic hydrogen flame.

Statement II: The heat generated during burning of these gases in oxygen is used to melt substances.