Mr. Brown, an French teacher, sat up far into the night ______ a paper on how to teach beginners of French.
A. and worked at
B. and worked on
C. working at
D. working on
work at 和work on 均有“从事于…”的意思，但前者隐含着“长期、努力”，而后者则含有“具体、临时”的意思，working on a paper 作伴随状语修饰 sat up.