Suppose Earth's Atmosphere Had No Greenhouse Gases

Suppose Earth's atmosphere had no greenhouse gases. Then Earth's average surface temperature would be

A) 15°C, or about the same as it is now.
B) - 16°C, which is well below freezing.
C) 20°C, or about 5°C warmer than it is now.
D) 0°C, or about the freezing point for water.
E) 10°C or about 5°C cooler than it is now.