l saw him in the library yesterday he --- a book at that momenta\ readsb\ is readingc\ was readingd\ will read说理由!

l saw him in the library yesterday he --- a book at that momenta\ readsb\ is readingc\ was readingd\ will read说理由!
l saw him in the library yesterday he --- a book at that moment
a\ reads
b\ is reading
c\ was reading
d\ will read
说理由!

l saw him in the library yesterday he --- a book at that momenta\ readsb\ is readingc\ was readingd\ will read说理由!
l saw him in the library yesterday.（有个句号） He --- a book at that moment.
答案选C
当时他正在看书.
或者：
l saw him in the library yesterday when he --- a book at that moment.
也是选C
过去进行时态.

C过去进行时态，表过去某时间正在发生的动作。