What does it mean when a compound chemical formula does not include any numbers

When a compound chemical formula does not include any number it means that compound has only one atom of the elements the compound is made up of.

For example, HCN is a compound made up of one atom of hydrogen, carbon, and nitrogen so it does not contain any number in its chemical formula.

The number present in front of any chemical formula of a compound signifies that there are that many molecules of the element is present in the compound. For example, 3O₂ contains 3 molecules of oxygen not 3 atoms of oxygen.

carbon

explanation:

the correct answer is 4) heat and pressure

metamorphic rocks arise from transformation of existing rocks types, in a process called metamorphism. during metamorphism the original rock(prolith) change into metamorphic rock by heating (150-200c) and pressure of more than 1000 bar. the original rock can be sedimentary, igneous, or existing metamorphic rock.