Only after you have reached eighteen the army.
A. you can join
B. can you join
C. you can join in
D. can you join in
网友回答
A. you can join
(答案→)B. can you join
C. you can join in
D. can you join in
解析：考查Only的部分倒装。当only放在句首，且强调状语的时候，句子使用部分倒装的形式。本题only放在句首，强调状语从句after have reached eighteen,故使用部分倒装的形式。另外参军使用join the army。故B正确。