Answer: A. Invalid.
When teaching the robot, the mode knob is hit in teach-in mode, in which the start signal from the external device is invalid.
In teach-in mode, when the mode knob is set to "teach-in", the robot will enter the teach-in state, at which time the start signal issued by the external device will be invalid and will not affect the movement state of the robot.
Therefore, the answer is A.