问题：
[多选] You work as an application developer at Certkiller .com. Certkiller .com has instructed you to create a class named MetricFormula. This class will be used to compare MetricUnit and EnglishUnit objects.The MetricFormula is currently defined as follows （Line numbers are used for reference purposes only）： 1. public class MetricFormula2. { 3. 4. } You need to ensure that the MetricFormula class can be used to compare the required objects. What should you do？ （）
A . Add the following code on line 1： ： IComparable {
B . Add the following code on line 1： ： IComparer {
C . Add the following code on line 3： public int Compare （object x， object y） {// implementation code }
D . Add the following code on line 3： public int CompareTo （object obj） {// implementation code }
