How can I put navbar tags in a row? [duplicate]

I use display: flex; and i need to put <li>s in a row. Here's the HTML

    <nav id='menu'>
        <ul>
            <li><a href='index.html'>Main</a></li>
            <li><a href='galery.html'>Galery</a></li>
            <li><a href="service.html">Service</a>
        </ul>
    </nav>

These <li> tags are under each other. I don't know how to put them in a row (i need to ensure compability between all browsers including mobile, so I use CSS' display: flex;).

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