In many big cities, the temperature is often 3 to 5 degrees higher than in the nearby countryside. This is called the "Urban Heat Island" effect. What is the main cause of this?
(A) There are too many people breathing in a small space.
(B) Concrete and asphalt (roads) absorb sun heat during the day and release it at night.
(C) Skyscrapers are closer to the sun, so they get hotter.
(D) City lights are left on all night, which warms the air.