Could someone answer this for me?

Voting.

Explanation:

Voting is the easiest way to make the government know what the people want. For example, voters can vote for politicians who would, in turn, vote for policies that the general public want. This is generally called indirect democracy, and is the most practiced version in US politics. The other way is direct democracy, in which the Citizens come up with enough support (usually in the form of petition & signatures), which in turn can be taken into account to make changes directly.

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demand is the need and oportunity to buy a given good or service.