1. The space shutle can travel at seven miles………. second.
A. every B. a C. for a D. each
I chose A but the answer is D. I don't find any difference between "each" and "every"
I would naturally say "... can travel at seven miles a second".
I would be more likely to say "can travel seven miles every second" (no "at"), rather than "can travel seven miles each second" or "at seven miles each second".
So, I disagree with the answer given. Someone else may have a different opinion.
In your other question I agree with the answer given but I'll leave it for a teacher to explain clearly.
not a teacher