_______you eat the correct foods____be able to keep fit and stay healthy
A Only if;will you B Only if;you will C Unless;will you D Unless;you will
答：A
译：只有正确饮食,才可能保持健康.
析：本题考查部分倒装.Only +状语,要引主句谓语的部分倒装（将you will be able to ...中第一个助动词will提到主语you的前面）,排除BDC
扩展：这状语有三种形式：
1.副词：Only then did he realized that he was wrong.
2.介短：Only in this way can you learn English well.
3.从句,常见的是when-时间状语从句：Only when the war was over in 1949 was he able to return home.
这里if-从句表示一条件,只有在这个条件下,你才可以保持身材,保持健康.