问题
选择题
.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
答案
答案：B
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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
答案：B