Four-fifths of the crop .A. are ruinedB. was ruinedC. were ruinedD. have been ruined

问题补充：

Four-fifths of the crop .

A. are ruined

B. was ruined

C. were ruined

D. have been ruined

答案：

A. are ruined

(答案→)B. was ruined

C. were ruined

D. have been ruined

解析：考察主谓一致。当分数百分数做句子主语的时候，谓语动词的单复数要根据分数百分数后的名词。如果名词是不可数名词孩子可数名词单数形式，谓语动词用单数，如果是可数名词复数形式，谓语动词使用复数形式。本题后面的名词the crop是可数名词单数，故谓语动词使用单数形式。故B正确。