Which CSS3 code segment styles the text color of EM elements to be red and semi-transparent?

A.
em { color: #ff00007f; }

B.
em { color: rgba (255,0,0,0.5); }

C.
em { color: rgba (255,0,0.127); }

D.
em { color: rgba (100%,0%,0%, 50%); }

Explanation:
http://www.w3schools.com/cssref/css_colors_legal.asp