this is true, but not many people know what an oligopoly is, so i went with monopoly
true, some dont follow the price trends, but consider the following issues
(1) price differences usually less than one peso
(2) price increase / decrease are all usually announced within 10 hours of each other
(3) the increment of increase / decrease are all usually the SAME
so while the setup is an oligopoly, these players act and behave like a monopoly - which is why the government must step in and MAKE sure that the prices implemented are in step with the world market.