Only by practising a few hours every day be able to master the language.A. you canB. wil

问题补充：

Only by practising a few hours every day be able to master the language.

A. you can

B. will you

C. you will

D. can you

答案：

A. you can

(答案→)B. will you

C. you will

D. can you

解析：only与状语连用放于句首时，句子的谓语动词须用倒装结构。be able to通常不和can 搭配使用。