题目内容：
Every few years, the coal workers.their lungs X-rayed to ensure their health.A. are having
B. have
C. have had
D. had had
参考答案：
答案解析：
单选题：Every few years, the coal workers.their lungs X-rayed to ens
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- 题目类型：单选题
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A. are having
B. have
C. have had
D. had had