Which CSS3 code fragment will style only the external links in an HTML document?

Which CSS3 code fragment will style only the external links in an HTML document?

A. a: visitied {font-weight: bold;}

B. a [href$= -http://-] {font-weight: bold;}

C. a {font-weight: bold;}

D. a [href^= -http://-] {font-weight : bold;}

Explanation: