Answer: b) Changes in the Earth’s axis cause the changes in Sahara's climate.
Explanation: The passage discusses the transformation of the Sahara desert over 11,000 years ago when it was once a green and vibrant landscape with lakes, rivers, grasslands, and forests. The change from a lush environment to the arid desert we know today is attributed to periodic fluctuations in the Earth's orbital axis. The slight wobbles in the tilt of the Earth's axis affect the angle at which solar radiation reaches the atmosphere, leading to alternating periods of humidity and aridity in the Sahara's climate. These fluctuations are known as African Humid Periods, during which more solar energy is received during the West African monsoon season, resulting in increased rainfall over North Africa. As the Earth's axis continued to change over time, it influenced the climate of the Sahara, causing it to transition from a green paradise to the arid desert it is today.