Xiaoli, a photography enthusiast, found that when black-and-white photos were developed in a dark room, the liquid column of the thermometer used was blue instead of the usual red.

Because the darkroom safety light is a red light, the red object can only reflect red light. If the sun shines on it, the other six colors will all be absorbed and only red light will be reflected. If the red light shines on it, all red light will be reflected, and only red can be seen, while the blue liquid column of the thermometer is black under the red light, so it can be seen more clearly.

Therefore, ABC． is selected.