There are two important factors to consider. Firstly, choose the subject combination that you have an aptitude in, that is, one that you can excel in. Your performance at the O-Level could be a good indication of your aptitude in the subject. Secondly, choose the subjects that you have a passion to find out what they offer at a higher level. As the A-Level curriculum is designed to be rigorous in content, having an interest in the subject is necessary to sustain through the course of study, The choice may not be a good one if either factor is not taken into consideration. You are strongly advised against choosing a subject because your friends have chosen it or because it is a popular combination.