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{exp:channel:categories channel="nieuws" style="linear"}
<input type="radio" name="category[]" value="{category_id}"  />  {category_name}

{/exp:channel:categories}

<input type="radio" name="category[]" value="{category_id}" {if category =="???"}checked="checked" />{/if}  {category_name}

{exp:channel:categories channel="nieuws" style="linear"}
<input type="checkbox" name="category[]" value="{category_id}" {if {selected}}checked=“checked”{/if} />  {category_name}

{/exp:channel:categories}

<select name="category[]" id="main_category" size="1">
    <option value="">-- Select a Category --</option>
    {exp:query sql="SELECT
                        c.cat_id,
                        c.cat_name,
                        p.selected
                    FROM
                        exp_categories AS c LEFT JOIN
                        (
                            SELECT cat_id, 'selected' AS selected FROM exp_category_posts WHERE entry_id='{entry_id}'
                        ) AS p ON c.cat_id=p.cat_id
                    WHERE
                        c.parent_id='0' AND
                        c.group_id='3'
                    ORDER BY c.cat_order ASC;"}
        <option value="{cat_id}" {if selected == "selected"}selected="selected"{/if}>{cat_name}</option>
    {/exp:query}
</select>