从A、B、c、D四个选项中,选出可以填入空白处的最佳选项,并在答题卡上将该项涂黑。
21.Although everything seems to have been taken into consideration, accident can happen when the astronauts walk in space.
A.不填;the B.an;the C.the;the D.an;不填
21.Although everything seems to have been taken into consideration, accident can happen when the astronauts walk in space.
A.不填;the B.an;the C.the;the D.an;不填

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