【题目】I was sleepy, so I shook my head and tried what I was doing.
A.to look forward to
B.focusing on
C.to concentrate on
D.devoting to

【答案】C
【解析】句意：我很困乏，所以摇头想集中精力于所做的事。A. to look forward to“期待”；B. focusing on “集中于”；C. to concentrate on“集中精力于”；D. devoting to“将......奉献给”。try to do sth.“尽力做某事”，此处用不定式。故选C。