Although everything seems to have been taken into consideration, accident can happen when the astronauts walk in space．
A. 不填；the
B. an；不填
C. the；the
D. an；the
网友回答
A. 不填；the
(答案→)B. an；不填
C. the；the
D. an；the
解析：第一空表泛指，并没强调是哪起事故，故用不定冠词an，in space 在太空上，为固定短语。句意：尽管一切事情都已考虑，但宇航员在太空行走时还是有可能发生事故。