单选题 Some people view the findings with caution, noting that a cause-and-effect relationship between passive smoking and cancer remains ______.
A. to be shown B. to have shown
C. to have been shown D. being shown

【正确答案】 A

【答案解析】[解析] remain to be shown“尚待揭示”，不定式的一般式表示要实现的情况，其完成式表示已发生的情况。remain doing“保持……；一直……”，如：He remained standing for one hour. 他一直站了一个小时。