用where完成下面句子

1) Mark twain is an american novelist whose works are very popular.
2) Oradee was studying at the university of hawaii in the early 1970s when he accidentally found out the technology of a milestone significance.
3) Pearl Buck, a well-known american novelist, read a lot of novels in her early childhood among which many belong to the works by Charles Dickens.
4) Although he racked his mind to give reasons for his playing truant, it seemed that nobody believed what he said.

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