Parts 1 and 2 in the Use of English paper consist of a text with gaps (missing words). In Part 1, you are offered four alternatives from which to choose the one that fits in the gap. Mostly all the alternatives fit grammatically, but only one makes sense. (A sentence such as I like to wear an elephant on my head, is grammatically correct, but it makes no sense). You need therefore to look carefully at the meaning of the whole sentence to see which word makes sense.