Make 'Pick an option' the default choice in product drop-down menus

pick-an-option.liquid

{% comment %}
  See https://docs.shopify.com/themes/customization/products/how-to-add-a-pick-an-option-to-drop-downs
{% endcomment %}

{% unless product.selected_variant %}
  {% if product.variants.size > 1 %}
    <script>
    var $addToCartForm = $('form[action="/cart/add"]');
    if (window.MutationObserver && $addToCartForm.length) {
      if (typeof observer === 'object' && typeof observer.disconnect === 'function') {
        observer.disconnect();
      }
      var config = { childList: true, subtree: true };
      var observer = new MutationObserver(function() {      
        {% for option in product.options %}
          $('.single-option-selector:eq({{ forloop.index0 }})')
            .filter(function() { 
              return $(this).find('option').size() > 1  
            })
            .prepend('<option value="">Pick a ' + {{ product.options[forloop.index0] | json }} + '</option>')
            .val('')
            .trigger('change');
        {% endfor %}
        observer.disconnect();
      });  
      observer.observe($addToCartForm[0], config);
    } 
    </script>  
  {% endif %}
{% endunless %}