error CS0120: An object reference is required to access non-static member `ScoreManager.score'

using UnityEngine; using System.Collections; using UnityEngine.UI;

public class ScoreManager : MonoBehaviour { public int score;

 Text text;
 
 void Start() 
 {
     text = GetComponent<Text> ();

     score = 0f;
 
 }

 void Update ()
 {
     if (score < 0f)

     text.text = "" + score;

 }

 public static void AddPoints (int pointsToAdd)
 {
     score += pointsToAdd;
 }
 public static void Reset()
 {
     score = 0f;
 }

}

 using UnityEngine;
 using System.Collections;
 
 public class CoinPickup : MonoBehaviour 
 {
     public int pointsToAdd;
 
 
     void OnTriggerEnter2D (Collider2D other) 
     {
         if (other.GetComponent<AstromanController>() == null)
             return;
 
         ScoreManager.AddPoints(pointsToAdd);
 
         Destroy (gameObject);
     }    
 
 }