Evidencecameupwhichspecificspeechsoundsarerecognizedbybabiesasyoungas6monthsold.

参考答案:

C

结构应用, 本句意为：六个月大的幼儿能够识别具体语音的证据已经有了。在本句中specific speech sounds are recognized by babies as young as 6 months old 是前面 evidence 的同位语， 因此应由that引导，答案应为C。