The best evidence that there is a critical period for language acquisition is the fact that

The answer that best completes the statement above is this: toddlers maintain a capacity to discriminate language sounds they have never heard. This is considered to be one of the best evidences for language acquisition. This is based on CPH or the Critical Period Hypothesis. This states that in the first years of life, humans use this time in order to develop language and readily use this but at the age of 5 and beyond, language acquisition becomes more challenging and difficult.