题文
Every possible means __________ to prevent air pollution, but the sky is still not clear.
| A．are being used |
| B．have been used |
| C．has been used |
| D．had been used |
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Every possible means __________ to prevent air pollution, but the sky is still not clear.
| A．are being used |
| B．have been used |
| C．has been used |
| D．had been used |
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