Metals of groups $7$,$8$,and $9$ do not form metallic hydrides. This is termed as

Which is the lightest gas?

Which one of the following methods is used to prepare carbon monoxide on a commercial scale?

Which property of interstitial hydrides is correct?

The hydrides of which group elements are examples of electron-precise hydrides?

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.