本题应该填 to be，似乎不可以填 as。think ...to be...中的不定式为宾补，是宾语从句的简略形式。比较：

People think the girl in the phote to be my sister.（不定式 to be my sister 是宾补）

= People think (that) the girl in the phote is my sister.（think后接的是宾语从句）

▲《葛传椝英语惯用法词典》在 think 词条有如下同义表达，没有用 as：

下面四句意思相同：

I think it a shame.

I think it to be a shame.

I think it is a shame.

I think that it is a shame.

第三句和第四句比第一句和第二句通俗。

▲另外在我在《张道真英语用法大全》中查到了以下用 to be 的例子，但没有用 as 的：

I thought (or supposed) the winner to be him. 我想获胜者会是他。

They thought these Japanese to be us Koreans. 他们认为这些日本人是我们朝鲜人。

▲比较: think of…as…句式（把……看成或当作……）：

We think of you as one (ie a member) of the family. 我们把你当成家里人。

He came to think ofItalyas his home. 他开始把意大利看成自己的家了。

The villagers hold them in awe and think of them as gods. 村民敬畏他们，认为他们是神。

He liked to think of himself as an avenging angel fighting for justice. 他喜欢把自己想作是为正义而战的复仇天使。

It is tempting to think of morality as a guide to human conduct. 把道德看成是人类行为的指导这种想法很值得考虑。