Aand b are substitute goods, but a and c are complementary goods. if the cost of producing a decreases, then the demand for

A and b are substitute goods, but a and c are complementary goods. If the cost of producing a decreases, then the demand for B will decrease and the demand for C will increase. The demand changes due to price and substitutes. If something is a direct substitute of something else and the demand for the item is the same and one is cheaper, the demand for the cheaper, same item will remain constant. 

Average cost of electricity per household

The price that a farmer asks for her watermelons.

Explanation:

Microeconomics is a branch of economics that studies the decisions individuals and firms make in response to changes in economic factors. These factors include price, resources etc. it studies how firms and individuals allocate and make decisions about resources

The other options are examples of macroeconomics.

Macroeconomics is a branch of economics that studies the economy as a whole. Macroeconomics studies economic aggregates such as inflation, unemployment, GDP and growth rate.