1. Initialize an array of 10 students and sort them by mark in ascending order. Use the interface System.IComparable.

2. Initialize an array of 10 workers and sort them by salary in descending order.

3. Define an abstract class Shape with abstract method CalculateSurface() and fields width and height. Define two additional classes for a triangle and a rectangle, which implement CalculateSurface(). This method has to return the areas of the rectangle (height*width) and the triangle (height*width/2). Define a class for a circle with an appropriate constructor, which initializes the two fields (height and width) with the same value (the radius) and implement the abstract method for calculating the area. Create an array of different shapes and calculate the area of each shape in another array.