yeah you should know at least 2 works from the mandatory topic (music from last 25 years) and 2 works form your additional topic really well. Cos they can ask you questions that involve comparing 2 works from each topic, or 2 works from one of the topics... they could ask you stuff such as compare and contrast using one or more of the musical concepts, or describe how unity and balance, etc... and also remember to memorise a few quotes to use as musical examples on the manuscript paper... it sucks how we only have 20 minutes to write an esay! thats so silly i reckon - theres just not enough time!!!