Since Venus rotates so slowly, we might be tempted to conclude that Venus, like Mercury, keeps one face always toward the Sun

Since Venus rotates so slowly, we might be tempted to conclude that Venus, like Mercury, keeps one face always toward the Sun. If this hypothesis were correct, we should expect that the dark side would be exceedingly cold. Pettit and Nicholson have measured the temperature of the dark side of Venus. They find that the temperature is not low, its value being only 29 degrees Fahrenheit, much warmer than our stratosphere in broad daylight. It is unlikely that atmospheric current from the bright side of Venus could perpetually heat the dark side. The planet must rotate fairly often to keep the dark side from cooling excessively.

1. What is the phenomenon being explained.?
2. What theories are given to explain the phenomenon?
3. Which theory seems the most plausible and why? (Use the criteria of adequacy.)