单选题 His parents ______ out this evening
A. all are B. are all C. both are D. are both

【正确答案】 D
【答案解析】[解析] both作同位语，谓语动词是系动词，both应放在系动词之后。all，both作主语同位语时，位于行为动词之前，be动词之后。