I've put the sentences in the order the answer key indicates they should go in.
S. A lie is when you say something happened which didn't happen.
Q. But there is only ever one thing which happened at a particular time and a particular place.
R. And there are an infinite number of things which didn't happen at that time and that place.
P. And if I think about something which didn't happen I start thinking about all the other things which didn't happen.
In this first example, you put SRQP. I think that makes sense, and we could understand what you're saying if you ordered them that way. But the order from the answer keys makes more sense to me because it flows better.
In the order from the answer key, we have a nice flow from talking about things which did happen to things which didn't happen, rather than jumping back and forth between those two ideas as we would if the order were SRQP.
Also, in the order from the answer key, we avoid starting sentence two with "and," which would be unusual since there is not a strong connection between sentence S and sentence R. (If we wanted to simply define what a lie is and then introduce the "infinite number of things" idea, we could simply start sentence R with "There are...," but since it starts with "And there are...," we need to take that into consideration when ordering the sentences.)
Q. Interdependence of food and labour market is important for the development process.
S. An upward shift in the food supply curve would simultaneously result in an upward shift in the labour demand curve.
P. The magnitude of the interdependence depends on the technique of production causing the shifts in the food supply curve.
R. Similarly, an upward shift in the food supply curve shifts up the food demand curve.
This case is much more confusing to me. I had to read this one many times to try to wrap my head around this one.
In your suggested ordering of QPRS, you set up a nice transition from the first sentence's idea of interdependence to a mention that the magnitude of interdependence varies. But in doing so, you've created a problem: You have sentence R, which discusses the food supply curve and beings with "similarly," placed after a sentence that does not discuss the food supply curve. So the sentences do not really make sense in that order.
The answer key's ordering, by contrast, sets up three consecutive sentences about interdependence between food supply and labor demand. Then it changes the subject to the relationship between food supply and food demand in the last sentence.
This is nice because we want to group similar ideas together to create a flow.
The reason this is confusing to me is because I think the wording of sentence R is weird. If we want to just mention after the interdependence discussion that, oh, by the way, food demand is also in play here, we could just say
An upward shift in the food supply curve also shifts the food demand curve up.
To me, the phrasing "Similarly, an upward shift..." just introduces confusion without improving flow at all: Similar to what? Food demand is affected by food supply similar to how labour is? Okay, yeah, but we are talking about the interrelation between food and labour, not between food supply and food demand.
And as a final note, there's an error in sentence Q. It should at a minimum say "markets," plural, if not "the food and labour markets."
Poor question, in my opinion.