Size of the background image

Not all of the standard templates, background image is stretched to full 
size monitor...

A good option if insert? - background-size:

1. 100% auto;
2. 100%;
3. cover;

What's the difference:     cover and 100%    ?

Which option is better?


Example:

/* Content
----------------------------------------------- */
body {
  font: $(body.font);
  color: $(body.text.color);
  background: $(body.background);
  background-size: ?

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