(a) Variable Cost: Variable costs are those costs which vary in amount with changes in the level of output or activity. Such costs increase or decrease in the same proportion in which the level of output increases or decreases. Variable costs vary in total amount but remain constant per unit of production. 

(b) Tidal Energy: The energy possessed by rising and falling water in tides is known as tidal energy. Tidal energy is harnessed for producing electricity by constructing a dam across a narrow opening to the sea. This is not a major source of energy. 

(c) Four main reasons: 

1. Communication gap. 

2. Population growth. 

3. Recognition of social responsibility. 

4. Education in Public Relations. 

(d) Balance Sheet: Balance sheet means a statement which shows the assets and liabilities of the organisation (including capital fund and other fund) on a particular date. It is a statement which reports the property values owned by the organisation and the claims of creditors and owners against these proper-ties. It depicts the financial position of the organisation on specified date. 

(e) The two essentials of a good advertisement: 

1. Prestige: This theme is used to popularize luxurious items like motor cars, refrigerators and furniture. Their possession acts as a symbol of prestige to their holders. 

2. Comfort: This theme is used to advertise products like fans, air conditioners, refrigerators etc., as such products help to make life comfortable.