单项选择题
We will not attack () we are attacked; if attacked, we will certainly counter-attack.
A．if B．when C．unless D．even if
单项选择题 "Do you hear someone knocking at the door" "Yes, I did. I heard him () three times."
单项选择题 The two thieves fled the town separately, () a bag.
单项选择题 A burning cigarette he threw into the wastepaper basket () fire to the hotel.
