The following figure shows two air-filled bulbs connected by a $U$-tube partly filled with alcohol. One bulb is painted black and the other is painted white (or silver/shiny). What happens to the levels of alcohol in the limbs $X$ and $Y$ when an electric bulb placed midway between the two bulbs is lighted?

• A
  The level of alcohol in limb $X$ falls while that in limb $Y$ rises.
• B
  The level of alcohol in limb $X$ rises while that in limb $Y$ falls.
• C
  The level of alcohol falls in both limbs.
• D
  There is no change in the levels of alcohol in the two limbs.