Namespace order c#.

Hi guys. Im having some problems here with using namespaces. I need this 3 namespaces. using System; //for Array using System.Collections; //For Random.Range and IEnumerators using System.Collections.Generic; // For List My problem is that If I'm deleting `System` than I can work with Random.Range and I can't find Array, if I'm deleting `System.Colections`, I can work with Arrays but I can't find `Random.Range and Ienumerator.` Is there a way to place this namespaces in a specific order or how should I face this problem?