问题
选择题
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
答案
答案：B
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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
答案：B
为单位脉冲函数，则积分
的函数值为（）。