surprised me most was such a little boy of seven could play the violin so well.
A. That...what
B. What...that
C. That...which
D. What...which
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A. That...what
(答案→)B. What...that
C. That...which
D. What...which
解析：考查名词性从句。What引导主语从句What surprised me most，并在句中做主语。That引导表语从句that such a little boy of seven could play the violin so well.因为这个表语从句很完整，所以使用that引导，that没有任何意思，只起连接作用。