Under what conditions would vacuum tubes be preferable to semiconductors? If the two are comparable in all other significant respects, such as maximum capacity. "Such as" is a key phrase here.
Remember that maximum capacity could be only one of several significant respects. Even if vacuum tubes become comparable in that respect, we don't know if they are comparable in all significant respects. Achieving a similar maximum capacity does not guarantee that vacuum tubes will be used in digital circuits. It only makes it possible that they could be used in digital circuits, depending on other factors. This is why B is incorrect.