7. The following table shows the marks of a test of 30 students in a class. Suppose the 5th student is set as the starting point and every seven student is selected to form a sample element. Class No. 01 02 03 04 05 06 07 08 09 10 12 13 14 15 Mark 65 52 57 55 61 47 44 157 49 61 78 70 60 59 45 Class No. 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 Mark 56 43 65 66 37 64 64 58 51 60 61 62 44 49 62 (a) What is the sampling method used? (2 marks) (b) Which students are selected in the class? (2 marks) (c) What is the average mark based on the students selected in the (b)? (2 marks)