简述青少年认知发展。
参考答案：
①皮亚杰的认知发展理论。
②维果斯基的最近发展区理论。
③青少年认知发展的特点。
By the time you have completed the essential training, you ________ exposed to virtually every new feature of the course . [ ]
A. will have been
B. are
C. have been
D. will be
脾为（）之本，（）源。
