单选题
Some people viewed the findings with caution, noting that a cause-and-effect relationship between passive smoking and cancer remained ______.
A、
to be shown
B、
to have been shown
C、
to have shown
D、
being shown
【正确答案】
A
【答案解析】
[解析] 本题空格处表达的意思是：……他们注意到被动吸烟和癌症之间的因果关系有待揭示。remain to be done意思是“有待于(被……)”，因此本题答案为A。
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