问题描述：
特殊句式
the scientists know only when a great deal more information has been obtained,__ possible to plan a manned trip to Mars.
A it will be
B will it be
C it is
D is it
the scientists know only when a great deal more information has been obtained,__ possible to plan a manned trip to Mars.
A it will be
B will it be
C it is
D is it
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