how to style individual paragraphs in css

CSS
/* This is the reference to style the above code of html in css*/

#one{
    font-family: 'Impact';
    color: red;
    font-size: 25px;
}
#two{
    font-family: 'Times New Roman';
    color: blue;
    font-size: 50px;
}
.three{
    font-family: 'Impact';
    color: red;
    font-size: 25px;
}
.four{
    font-family: 'Times New Roman';
    color: blue;
    font-size: 50px;
}
<!--This is an example of html problem-->

<!doctype html>

<html>
    <head>
    <link rel="stylesheet" type="text/css" href="test.css">
    </head>

    <body>
    <p id="one"><p>Paragraph 1</p>
    <p id="two"><p>Paragraph 2</p>
    <p class="three">Paragraph 3</p>
    <p class="four">Paragraph 4</p>
    </body>
</html>
Source

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