Hardening of oils is caused by

Which of the following cannot be reduced by hydrogen?

What is obtained by the reaction of $HCOOH$ with concentrated $H_2SO_4$?

How many of the following statements is/are correct?
$(a)$ Mercury is the only metal that exists as liquid at room temperature.
$(b)$ Among non-metals,carbon has the highest melting point.
$(c)$ Hydrogen is the most abundant element in the universe.
$(d)$ Oxygen is the most abundant element in the earth's crust.

The action of water or dilute mineral acids on metals can give:

The maximum sum of the number of neutrons and protons in an isotope of hydrogen is