What is the difference between the following line-height settings?

line-height: 20px
line-height: 2

• The value of 20px will set the line-height to 20px. The value of 2 will set the line-height to twice the size of the corresponding font-size value.
• The value of 20px will set the line-height to 20px. The value of 2 is not valid.
• The value of 20px will set the line-height to 20px. The value of 2 will default to a value of 2px.
• The value of 20px will set the line-height to 20px. The value of 2 will set the line-height to 20% of the corresponding font-size value.