What is meant by the term opportunity cost?

A bank faced with the wholesale loss of deposits is likely to shut down despite fundamentally sound balance sheet. Why could this be?

A) Banks have accountants that are too optimistic. B) Banks purposely lie about their balance sheets in order to attract more clients. C) Many bank assets are illiquid and cannot be sold quickly to meet deposit obligations without substantial loss to the bank. D) Many banks operate on a budget that exceeds their actual reserves. E) Many banks will shut down to preserve their interest profits.

In some developing countries and in some former Communist countries, people exchange their domestic currencies for foreign currencies such as the dollar in black markets. Why would this practice go on?

What will be an ideal response?