Sometimes simplest is best. The question only asks for one example, and that's what's been given. If the question had asked for two examples, I think that would have been worth more than 4 marks. Some marks are given for what is invisible, like ingenuity. Only the results of a thought process can be written down, and that thought process could take up a lot of time, but result in a short answer. You know how best to judge how the marking works. Let see what your teachers think! Best wishes.