The best answers would be "no substitutes", "resources used inefficiently", and "barriers to entry". For no substitutes, if a certain company has a monopoly over a certain product that no one can get anywhere else, then obviously there are no substitutes for that product. Take gas for example, in some countries, gas is monopolized by a big company. Since there are no substitutes, everyone has to buy from the company. Because of the size of the company, other smaller competitors would not dare enter. This is known as a barrier of entry. Also, since monopolies control certain products, they do not maximize their resources to optimize productivity because consumers are at their mercy anyway.
