While I _____________, someone ___________at the door.
| A．did my homework; knocked |
| B．do my homework; knocks |
| C．was doing my homework; knocked |
| D．was doing my homework; knocks |
While I _____________, someone ___________at the door.
| A．did my homework; knocked |
| B．do my homework; knocks |
| C．was doing my homework; knocked |
| D．was doing my homework; knocks |
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