l saw him in the library yesterday he --- a book at that moment

问题描述:

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.(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.