题文
The research is so designed that once __________ nothing can be done to change it.
| A．begins |
| B．having begun |
| C．beginning |
| D．begun |
推荐试卷
The research is so designed that once __________ nothing can be done to change it.
| A．begins |
| B．having begun |
| C．beginning |
| D．begun |
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