In those days books could only be produced one at a time .A. in handB. with hand　C. by ha

问题补充：

In those days ,books could only be produced one at a time .

A. in hand

B. with hand

C. by hand

D. by hands

答案：

A. in hand

B. with hand

(答案→)C. by hand

D. by hands

解析：in hand在手头，在控制中；with hand用手；by hand用手，手工的；by hands无此说法。句意：在哪个时代，书籍只能手工一本一本地写。With一般表示用某种工具，by表示用某种方式，结合语境可知本句中强调书籍的生产方式，故选C。