The word critical could best be replaced by.

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 43 to 50.

In young language learners, there is a critical period of time beyond which it becomes increasingly difficult to acquire a language. Children generally attain proficiency in their first language by the age of five and continue in a state of relative linguistic plasticity until puberty. [A] Neurolinguistic research has singled out the lateralization of the brain as the reason for this dramatic change from fluidity to rigidity in language function. Lateralization is the process by which the brain hemispheres become dominant for different tasks. The right hemisphere of the brain controls emotions and social functions, whereas the left hemisphere regulates the control of analytical functions, intelligence, and logic. [B] For the majority of adults, language functions are dominant on the left side of the brain. [C] Numerous studies have demonstrated that it is nearly impossible to attain a nativelike accent in a second language, though some adults have overcome the odds, after lateralization is complete. [D]

Cognitive development also affects language acquisition, but in this case adult learners may have some advantages over child learners. Small children tend to have a very concrete, here- and-now view of the world around them, but at puberty, about the time that lateralization is complete, people become capable of abstract thinking, which is particularly useful for language. Generally speaking, adults can profit from grammatical explanations, whereas children cannot. This is evidenced by the fact that children are rather unreceptive to correction of grammatical features and instead tend to focus on the meaning of an utterance rather than its form. However, language learning theory suggests that for both adults and children, optimal language acquisition occurs in a meaning centered context. Though children have the edge over adult language learners with respect to attaining a nativelike pronunciation, adults clearly have an intellectual advantage which greatly facilitates language learning.