well i would either go to A or C.
I would pick A if i were you it is more funny.
simple space for one also aeronautics, engineering, physics ect.
Frontier is defined as 1) a line separating two countries, 2) the extreme limit of settled land beyond which lies wilderness, or 3) <span>the extreme limit of understanding or achievement in a particular area.
Right away we can cross out B because the wilderness is pretty close to a frontier. We can also cross out D because it doesn't make sense with the definitions given. So now we are left with A and C.
A is a funny misuse of the word; they probably meant to say "front." So, now we only have C. C is correct, and it makes sense because space does have an </span>extreme limit of understanding in science.
