Keynesian economics?

[Samantha]

'In applying ISLM analysis to show the Keynesian route to lift an economy out of recession, the assumption that general price level remains unchanged might seen reasonable. In relaxing this assumption a boost in AD may affect both quantity and price. Show the relevance of AS to resolving the issue of whether a boost to AD is likely to reduce unemployment or lead to price inflation.'

Hi the problem I am having is understanding the first section of the question. The only reason I can think of for a fixed price level is if the LM curve is horizontal, is there anything I'm missing?

Thankyou.