When Rianna was 11 months old, she was injured in a car crash. She was in a coma for almost a year and then spent many months of recovery from a brain injury. By her third birthday, although she is making good process, she has not yet spoken, and her parents are fearful that she will never speak. The doctors, however, feel confident that Rianna will learn to speak. This is because the first four years are not a _____ for language learning.