Ok look guys think about it, what is sucrose?
Its a carbohydrate
What happens to sucrose when its is changed by enzymes
It becomes an individual glucose molecule
What happens when an organism (An Aphid in this case) has an excess of carbohydrates
They are stored as Fats
And so if the aphid is eating the phloem it will rightly obtain sucrose
Which can than be stored if in excess as fats
Thus meaning that fats cannot be the answer
Allowing us to conclude through elimination of the obvious first two and now Fats
That water is to be lacking.
Am i right... Think about it 
Nice Explanation but dont think so
glucose can be changed into glycogen and stored 
And what if it didnt feed on excess of carbs?!!!
Yeah true but the question clearly states that the contents of the phloem are all that the aphids eat
So therefore we know that it is eating usually excessive amounts of carbs thus justifying what i said.
Yeah but glucose is only changed into glycogen as a means of homeostasis, thats a completely different answer or even question
And plus isn't it the xylem's job to carry water and minerals.
Sucrose can be used as a transporting agent for amino acids.
Think about it
Haven't you ever felt a sticky substance when you cut open a leaf, if you haven't try it.So what i am trying to say is fats are just basically carbs stores assuming as said before that aphids are only eating the contents of the phloem as the question states.
