Preferably speaking, you are given more marks for evaluation so you cant really hope for something to come up more, for instants if you are hoping for a general gender question you will have to write more for evaulating gender with ethnic and class inequality, or evaluate it with other theories. If its a question aiming towards a specific theortical approach for example the weberian explanation of say for instants ethnicty, you will be talking less about weberianism and more or less writing more in an evaluation. So basically it doesnt matter if you know little about the theory the question is asking about but you must be famillar with a wider range of approaches to evaluate with, this gets you more marks. There is alot more to say about class in my opinion, thats why i think OCR favour it. It also doesn't look like age will appear so my best guess is to focus on just the remaining three topics. As for 15 and 25 markers my best guess is it will be based on a research method that either hasnt appeared in while, or has never come up before.