This year M.Sc. Students will have an option of answering a section either on Maths or on Economics. So I am opting for Eco as I am familiar with it. At this moment I suggest you to make a choice between these two & start preparing.
Answer to your question is an easy inference problem. If you have read the PDF document attached by Andrea carefully, the following is mentioned:
"Note: If you are found to have answered questions from both Tests IIIA and IIIB, then only the Test with the higher number of answered questions will be graded. If equal number of questions are answered from both IIIA and IIIB, then only the first Test i.e. IIIA will be graded."