Answer:
The characteristics which is shared among both monopolies and oligopolies is that they have significant barriers to the entry into the market.
Step-by-step explanation:
Oligopoly market is the market structure which have a small number of firms, and could not have significant influence. The market have the barriers for entering into the market.
Monopoly market is the one which have a market structure having a single seller, selling the unique product, faces no competition and no substitute available with customers. In this market, there is also barriers for entering into the market.