He has worked __________ the maths problem for nearly an hour but he simply can't work it __________.
| A．on；at |
| B．on；out |
| C．at；up |
| D．for；out |
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He has worked __________ the maths problem for nearly an hour but he simply can't work it __________.
| A．on；at |
| B．on；out |
| C．at；up |
| D．for；out |
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