题文
.The other day I was showing the boys the weekly chemistry experiment when, ________ I knew it, the mixture was bubbling over everywhere.
| A．if | B．before | C．though | D．unless |
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.The other day I was showing the boys the weekly chemistry experiment when, ________ I knew it, the mixture was bubbling over everywhere.
| A．if | B．before | C．though | D．unless |
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