从A、B、C、D四个选项中，选出可以填入空白处的最佳选项。
21.________________, I was so careless that it got loose and was hit by a car.
A . While I walked the dog B. While I walking the dog
C. While walking the dog D .While I am walking the dog .
22. It was the first time that I_____________ the night face to face .
A. had seen B. have seen C. saw D. see
23. Although I try to talk to my classmates, I still find _______ hard to make good friends with them.
A. them B. they C. it D its
24. Native English speakers can understand each other __________ they don’t speak the same kind of English.
A. as if B. even if C. if D. unless