Which is the most important ore of iron?

Select the correct option using $T$ (true) or $F$ (false) for the following statements:
$(1)$ Calamine and Siderite are carbonate ores.
$(2)$ Zinc blende and Pyrite are sulphide ores.
$(3)$ Haematite and Magnetite are hydroxide ores.
$(4)$ Malachite and Cuprite are copper ores.

What is the formula of pyrolusite ore?

Three most occurring elements in the earth's crust are

What percentage of the Earth's crust by mass is composed of oxygen (in $\%$)?

What is a substance called from which an element can be economically extracted?