Answer:
This is because the first four years are not a critical period for language learning
Explanation:
According to the Critical Period Hypothesis (CPH) which states that the first few years of life constitute the time during which language develops and after which sometime between age 5 and puberty language learning becomes much more difficult and ultimately less successful.
Since Rianna just turned 3, she has not reached the critical period for language learning and can still learn how to speak.