21、Every few years, the coal workers，        their lungs X-rayed to ensure their health．

A．a(chǎn)re having                                      B．have

C．have had                                     D．had had

21、B

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Every few years, the coal workers，        their lungs X-rayed to ensure their health．

A．a(chǎn)re having                                      B．have

C．have had                                     D．had had

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