For a Pair of Stars to Be Classified as an Optical

Multiple Choice

For a pair of stars to be classified as an optical double, which one of these conditions must be true?

A) The stars must lie in almost the same direction from Earth but must not be orbiting around each other.
B) The stars must be orbiting around each other, and one must periodically cross in front of the other (i.e., it must eclipse the other) as seen from Earth.
C) The stars must be orbiting around each other, and absorption or emission lines from both stars must be visible in the spectrum.
D) The stars must be orbiting around each other, and both stars must be visible through telescopes from Earth.