The heat island effect, or urban heat island effect, is when an isolated region of land is significantly warmer than its surroundings due to human activities. These activities include the usage of materials that are dark or absorb significantly large amounts of heat. These materials retain the heat that is incident on them; therefore, the entire region exhibits a significantly elevated temperature when compared to an area with less buildings and more vegetation. Therefore, the answer is B. the materials used in building cities retain heat.