Sodium(na) is a highly reactive metal and chlorine is a poisonous gas. which compound do they form when they chemically combine?

Sodium lose one electron and convert into sodium ion Na+

Chlorine lose one electron and form Cl- ion

When these ions come together they from an ionic compound NaCl,

table salt

The ionic compound which consist ionic bond. The ionic bond is a type of bond which formed between positive and negative ions or metal, sodium and non-metal , chlorine  

c- cold periods in earth's history during which glaciers covered much of the surface.

explanation: