How do people solve a level bio past papers

Here is a pic of the question


My question is :
When do I know when to state the effect and explain why it happens :
In this example I would say that higher levels of exercise require more energy so the rate of respiration of muscle cells increases and this causes the breathing rate to increase etc etc etc

And when do I state the effect and say how it happens
Ex: higher levels of exercise increase the breathing rate and depth of breathing.
When you exercise conc of CO2 and lactic acid in the blood increase and this is detected by chemoreceptors in the medulla oblongata which sends impulses to the respiratory center in the medulla oblongata etc etc etc ?