One day humans ..................on Mars if US President Bush's recently announced plans ...............to fruition.

a- could be walking - come

b-would walk - had come

c- can be walking - will come

d- would have wlked -come

e- will be walking - might come

According to you , which is the best for the blanks in the sentence above? please explain

could = it's possible under certain circumstances. It depends on Bush's plans coming to fruition.
The best choice is letter e because it indicates that this will happen in the future. The others have a past or present tense for at least one of the blanks.
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same reading here (a)
what about the choice E?
E isn't good
"might" is too tentative to establish the certainty required by "will"
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Agreed - "will" and "might" don't work together.
thanks thanks Emotion: smile

a is correct

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