It may be another fifteen years before spacecraft from Earth again

1. First and foremost, the sentence starts by saying that it "may" be... implying that whatever is gonna be written is someone's imagination or speculation or a guess.
When that happens you CANNOT use "will". If the sentence were to read the following:
Fifteen years from now, a spacecraft from earth "will" venture to Mars.

This sentence is correct because there is certainty.

Based upon this reasoning you can eliminate C and E.

2. Again and Now . both are adverbs and the placement of adverbs is also crucial. In D, what is "now" modifying? Known or to be? These are called squinted modifiers. After reading the sentence fully, yes, we can say that it is modifying known.. but it is not that clear gramatically. YOu can read more about squinted modifiers: https://gmattoughies.blogspot.com/2009/0 ... inted.html
So D is also out.

3. We are left with A and B. Some of you might say that B is incorrect as the modifier "again" should be as close to venture. Well, not all that true. I can give example: Will you spell your name again, please?
Here again is modifying spell, but look at its placement.
So there's something else that makes B incorrect.

4. If you saying that B is incorrect because it uses "known for" and not "known to" as the correct idiom.. I can give you another example
Ex 1: Which country is known for its wide use of sauces with its food?
Ex 2: Best known for its use as a mild antidepressant, St. John's wort is also being studied for its possible affects on other mood disorders such as anxiety and obsessive-compulsive disorder.

So usage of "known for" doesn't seem to be an issue.

Is "known for being" wrong? Again, there are examples to show that this is also correct
Ex 1: President Bush is known for being extremely profane.

So what is the problem with this sentence?


In the context of the whole sentence, the use of "known for being" and "known to be" is the key.

Known to be cold, dry and lifeless means that we are aware that mars has the characteristics as stated.

Known for being means that Mars is known as a planet that is cold, dry and lifeless..

Please re-read the sentences and you'll notice the subtle difference the meaning conveyed.

and that's the only difference that makes me choose A as the answer.