Can someone explain the solution to this question?

[cbplayer]

It is desirable to expand the yield of a harvest only when________________additions in time,exertion, and other variable factors of production are not also required .

choices :

predestined 
commensurate 
analogous
deliberate 
indeterminate

I read an explanation from the Princeton Review book but I still don't get why commensurate is the answer. I understand that idea behind this sentence is that expanding the yield of harvest is desirable if it doesn't require extra work but I can't see why one would pick commensurate or a similar word as the correct word. I mean, through process of elimination, I might be able to end up with commensurate as the answer but I would never have thought that it would be the right word. When I was trying to come up with a word, I thought "several" would be a good word. Am I missing something?

[SAH]

"Commensurate" can be read as "proportional" or "corresponding".  You could talk the others into making sense, but "commensurate" is the only choice that fits without additional explanation.  

[Vince Kotchian GRE Prep]

It's a badly written question in that two answers (commensurate, analogous) could be correct. I try to steer students away from third-party-written verbal material for this and many other reasons.