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; }