Final answer:
The statement about rifts slowly filling with rainwater is true as geological processes create depressions like rifts, which can collect rainwater and contribute to lake formation, as seen in structures such as the Eastern Rift on Earth.
Step-by-step explanation:
The statement 'When some of the rifts developed they slowly filled with rainwater' can be considered true. Rifts or basins are geological structures that can form when a crust cracks and moves apart. As erosion and geological processes create these depressions in the Earth's crust, they can collect rainfall leading to the formation of lakes and other water features. For example, in the Eastern Rift, which is a valley or basin with shallow lakes, the absence of outlets and high evaporation rates lead to varying levels of water composition, from freshwater to extremely alkaline. Moreover, evidence from the Mars Global Surveyor's observations of young gullies and recurring slope lineae suggests that water, whether on Earth or other planets, plays a significant role in shaping the terrain by flowing into these rift-created basins and interacting with the geology of the area.