def _selected_value(browser, select_css_selector):
    """
    Also checks if exactly one option is selected.

    Parameters
    ----------
    browser
        Splinter browser
    select_css_selector
        CSS selector for the <select> element

    Returns
    -------
    Value of the selected option.
    """
    select = browser.find_by_css(select_css_selector)
    selected_options = select.find_by_css('option:checked')
    assert len(selected_options) == 1
    selected_option = selected_options.first
    return selected_option.value