Which gas can be adsorbed on a metal and released when required?

Which of the following is $NOT$ the correct order of reactivity of metals?

Name a non-metal which is lustrous and a metal which is non-lustrous.

Which gas has the potential to become a major energy source in the future?

$(a)$ Why does calcium start floating when added to water?
$(b)$ Most metals do not release hydrogen gas when reacting with nitric acid. Why?
$(c)$ Write the chemical equation for the reaction of iron with steam. Name the compound of iron obtained.

Generally,when metals are treated with mineral acids,hydrogen gas is liberated,but when metals (except $Mn$ and $Mg$) are treated with $HNO_3$,hydrogen is not liberated. Why?