it seems------the boy knew everything,though in fact he does

我认为选bthat
it seems 后面可以接的形式大概有三种：第一种“it seems that...”意思上表示的是“看上去,似乎”,后跟陈述语气的句子.也可以转换成 名词或代词 ＋ seem ＋ 动词不定式的句型,其原意不变.如果动词不定式为to be ＋ 形容词时,to be往往省略.
EG:It seems that they don't like the idea.他们似乎不喜欢这个主意.
-----=They don't seem to like the idea.
第二种“it seems to sb”中it是个代词,代替前文所说的主语,后一般接句子.来表达某个人对某个问题的感受.意为“在某人看来.”
懂了吗