[求助]如何判断呢

140. It may be another fifteen years before spacecraft from Earth again venture to Mars, a planet now known to be cold, dry, and probably lifeless.
(A) again venture to Mars, a planet now known to be
(B) venture to Mars again, a planet now known for being
(C) will venture to Mars again, a planet now known as being
(D) venture again to Mars, a planet that is known now to be
(E) will again venture to Mars, a planet known now as being

我觉得A和D都可以.D错在哪里呢?答案是A

1，A，强调再次venture to Mars；现在被认为。意思合理。   D，强调再次to Mars，和A的意思有所不同；被认为现在是。意思有变化，而且不合理。

2，known to be + n不可以，要用known as +n。但本题的known to be + adj是可以的，所以A没有问题。

请指正。

dashasha mm:看不懂你的问题。

before后面可以/应该是个什麽结构， 应该可以是句子或名词。

如果是句子，那是不是应该before aircraft again ventures to mars. (dashasha mm:is this your question?, I have the same concern)

如果是(E) will again venture to Mars, a planet now known to be, 是不是比(A) again venture(s) to Mars, a planet now known to be 好? I mean in here why do not use 将来时？

1，sheep和spacecraft都是单复数同形。

2，before，when的从句中可以用将来时吗？哪儿有这种例外？