Merkle Graduate PPC Marketer interview questions

Had an initial 30 minute phone interview. This delved deeply into my past experience, academic record, even asking why I'd chosen certain subjects at school. Also asked some detailed questions to test my knowledge of ppc marketing. I'd recommend reading all of Adwords' foundation level exam guide to prepare for this. Feedback was good, but they recommended I study up on PPC a little further. I progressed to the next level, an Assessment Centre. There were about 8 of us there. The day started with them (Periscopix) giving a presentation on the company. Make sure to get in there with a smart question after this. We then gave short presentations about ourselves and one aspect of marketing we'd had to prepare ahead of time. A few people just weren't very good at this- didn't speak clearly enough, no eye contact, just read from notes etc. There was then a group exercise ('lost at sea'- what do you take on your raft?) Which was slightly ruined by people in the group having done it before. I just made sure I participated. At the end, they kept 4 of us back for some extra questions- an additional 10 minute interview which wasn't on the schedule. This is where I got too nervous. The feedback was that I performed well in the preceeding bits, but my motivations for ppc from this bit weren't clear. Very frustrating as I was really keen!