What is a blank keyframe?

A. a frame that is the same as the keyframe before it - it looks like a gray rectangle without a dot

B. a frame that represents a change in content, be that scale, movement, rotation, color, etc - it looks like a gray rectangle with a black dot.

C. a frame without any content - it looks like a white box

D. a frame without any content - it looks like a white box with a white dot