checked attribute, exclusively usable with
type="radio" and the
type="checkbox", is a boolean attribute ( you can check the challenge about the
required attribute, boolean aswell, at this link: use html5 to require a field).
As boolean attribute you simply have to add the word
checked to the
input HTML element you want to set checked by default (the challenge requires you to set by default the first of the checkbox and radio-buttons already present in the code).