Question: Given the following HTML, how could we use one line to hide or show the button?

1. `$('.btn-primary').toggle();`
2. `$('.btn-primary').showHide();`
3. `$('.btn-primary').not(':visible').show();`
4. `$('.btn-primary').css({ display: 'block'});`

Answer: The correct answer of the above question is Option A:`$('.btn-primary').toggle();`