In the troposphere, the lowest level of the earth' s atmosphere, the temperature decreases as one progresses straight upward. At the top, the air temperature ranges from -50 degrees Celsius over the poles to -85 degrees Celsius over the equator. At that point the stratosphere begins, and the temperature stops decreasing and instead increases as one progresses straight upward through the stratosphere. The stratosphere is warmed by ozone. When an ozone particle absorbs a dose of ultraviolet sunlight, heat is generated. If the statements above are true, which one of the following must also be true? ['The troposphere over the poles is thicker than the troposphere over the equator.', 'The temperature at any point at the top of the stratosphere is at least as great as the temperature at the top of the troposphere directly beneath that point.', 'The temperature in the middle part of the stratosphere over the North Pole is at least as great as the temperature in the middle part of the stratosphere over the equator.', 'It is warmer at the top of the stratosphere over the poles than it is at the top of the stratosphere over the equator.']