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Earth's atmosphere acts like a blanket, trapping some of the heat received from the Sun and keeping the planet warm enough to sustain life. This phenomenon is known as the greenhouse effect. Without an atmosphere, this heat-trapping mechanism would not exist, and the planet's temperature would rely solely on the amount of sunlight it receives.

With the atmosphere, Earth currently has an average temperature of about \(15^{\circ} \mathrm{C}\). Without the atmosphere's greenhouse effect, though, the majority of the heat from the Sun would be lost to space. As a result, the average temperature on Earth's surface would be significantly lower. Now, we can choose the correct option given the information we've discussed: (A) Lower - This option is correct, as we've learned that the average temperature on Earth's surface would be lower without an atmosphere due to the absence of the greenhouse effect. (B) Higher - This option is incorrect, as the lack of an atmosphere would cause Earth's surface temperature to drop, not rise. (C) Same - This option is not correct, as we've learned that the atmosphere has a significant impact on Earth's average temperature. (D) \(0^{\circ} \mathrm{C}\) - This option is not correct, as the temperature would be lower than the current average, which is well above 0 degrees Celsius. So, the correct answer is: (A) Lower