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

I am adding scoring to my Tetris game and received this error, Assets/Groups.cs(79,17): error CS0120: An object reference is required to access non-static member `Groups.numberOfRowsThisTurn'. I have tried to look it up online but I still can't figure out what the problem is. Please help.

Here is my code:

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

public class Groups : MonoBehaviour {

 public static int gridWidth = 10;
 public static int gridHeight = 20;
 public static Transform[,] grid = new Transform[gridWidth, gridHeight];

 public int scoreOneLine = 40;
 public int scoreTwoLine = 100;
 public int scoreThreeLine = 300;
 public int scoreFourLine = 1200;

 public Text hub_score;
 private int numberOfRowsThisTurn = 0;
 private int CurrentScore;

 void update() {
     updateScore ();
     updateUI ();
 }
 public void updateUI() {
     hub_score.text = CurrentScore.ToString ();
 }
 public void updateScore(){
     if (numberOfRowsThisTurn > 0) {
         if (numberOfRowsThisTurn == 1) {
             clearedOneLine ();
         } else if (numberOfRowsThisTurn == 2) {
             clearedTwoLines ();
         } else if (numberOfRowsThisTurn == 3) {
             clearedThreeLines ();
         } else if (numberOfRowsThisTurn == 4) {
             clearedFourLines ();
         }

          numberOfRowsThisTurn = 0;

     }
 }
 void clearedOneLine(){
     CurrentScore += scoreOneLine;
 }
 void clearedTwoLines(){
     CurrentScore += scoreTwoLine;
 }
 void clearedThreeLines(){
     CurrentScore += scoreThreeLine;
 }
 void clearedFourLines(){
     CurrentScore = +scoreFourLine;
 }

 public static Vector2 round(Vector2 v) {
     return new Vector2(Mathf.Round(v.x), Mathf.Round(v.y));
 }

 public static bool isInsideGrid(Vector2 pos) {
     return ((int)pos.x >= 0 &&
         (int)pos.x < gridWidth &&
         (int)pos.y >= 0);
 }

 public static void Delete(int y) {
     for (int x = 0; x < gridWidth; ++x) {
         Destroy(grid[x, y].gameObject);
         grid[x, y] = null;
     }
 }

 public static bool isFull(int y) {
     for (int x = 0; x < gridWidth; ++x) {
         if (grid [x, y] == null) {
             return false;
         }
     }
     numberOfRowsThisTurn ++;        //<---- error here

     return true;
 }