# Entropy of a system as universe expands

#### Halcyon32

2+ Year Member
I just encountered this problem in my EK book and am a bit confused:
All of the following are examples of processes which increase system entropy EXCEPT:

A. the expanding universe
B. aerobic respiration
C. melting ice
D. building a bridge

I initially said D (which is correct) but then I saw that the question specified entropy of a system so I put A. My reasoning was that the expanding universe would increase the entropy of the universe and not the system and figured it might have been one of those "so obviously not the right answer that it is the right answer" type questions. Why would the increasing entropy of the universe increase the entropy of a system, which I'm assuming is independent of the universe?

#### Neuroplasticity

2+ Year Member
I believe the system in choice A is the universe.

#### popopopop

7+ Year Member
I think D is just a clearer cut answer. Building a bridge from parts will decrease the entropy/number of parts to one structure. You can reason it out that D is a safer answer than A if you're down to 50/50. It never stated the universe was or was not the system, so you're making an assumption not stated in the question. I think that was the intended lesson from this question.

OP
H

#### Halcyon32

2+ Year Member
Ok that totally makes sense. Thanks guys!