从SELECT control tagged multiple接收所有选定选项

我有一个表单,它的select控件标记为multiple。但当我收到值时,它只是第一个选定的值,而不是我期望的多个值。我想是“1,4,2”

HTML格式:

<form>  
        <select id="responsible" name="responsible" multiple="multiple">
                <option value="1">Bare hyggelig</option>
                <option value="3">BUA &#xD8;stensj&#xF8;</option>
                <option value="2">Et Levende Bogerud Beboerforening</option>
                <option value="4">Vennegruppe V&#xC5;R</option>
        </select>
</form>

行动:

public async Task<IActionResult> CreateActivity([FromForm] string responsible)
{
    return RedirectToAction("Calendar");
}