Answer
Two things are important: 1) The red algae lack flagella and need large amounts of water for gamete transfer, while the green algae have flagella and can operate in small volumes of water which could be found on land; 2) The green algae have UV protectants which the red algae lack, making them and their descendants among the land plants better able to survive in the higher-UV environment.
Work Step by Step
Review the material on both groups and look for things which connect to water. You may want to jump ahead in the book and skim material on the land plants to see how they deal with the amounts of water available on land— you will see the swimming sperm of things like mosses moving in thick films of water.
