If the prefix ex- means "out," the exact meaning of the word exposed is D. driven out
To pose means to present something. So, if we add the prefix -ex and take the literal meaning, to expose means to bring something out in the open or in danger.
Because it was a rare element, visually pleasing (used in jewellery), and could be sold for lots of money. It is also very unreactive so it can be used for many purposes (eg. in computers)
