at which infants smile when spoken to,say "a-goo",bebble and coo 是the age的定语从句.
全句翻译：他们说婴儿在有人跟他们说话时会微笑、会说“啊咕”、波波和咕咕的年龄预示着将来的发展,并且可能是指向学习的重要线索.
语法成分分
{主语They} {谓语say} {宾语从句that [主语the age] [定语从句at which (主语infants) (谓语smile) (状语从句when spoken to) ,(谓语say) (宾语"a-goo",bebble and coo)] [谓语foreshadow] [宾语later development] and [谓语may be] [宾语important clues] [定语pointing to learing]}.
希望大中小括号的层次{ [ ( ) ] }还算清晰
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