问题
选择题
To those scientists, it still remains to be a question ______ man can go back to the old days with time machine.
A．if
B．that
C．how
D．whether
答案
D
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To those scientists, it still remains to be a question ______ man can go back to the old days with time machine.
A．if
B．that
C．how
D．whether
D