Economics homework im stuck... could you please help!?

[I Lost My Nickname!]

9) For which market structure will the market result in the efficient outcome?
A) monopoly
B) pure competition
C) monopolistic Competition
D) all of the above

10) The industry for discount department stores such as Wal-Mart and K-mart would be characterized as:
A) monopoly
B) oligopoly
C) monopolostic competition
D) pure competition

20) If firms in an industry form a cartel, the industry will sell a ____ quantity and charge a ______ price than if the industry were characterized by pure competition.
a) higher;lower
B) lower;lower
C) higher;higher
D) lower;higher