Bowling Game Kata int[] to List<int>

In the Bowling Game Kata we used a primitive int[] and all the cool kids are using generic List<int>.

BowlingGame/Game.cs
using System.Collections.Generic;

namespace BowlingGame
{
    public class Game
    {
        private readonly List<int> rolls = new List<int>(21);
        private int score;

        public void Roll(int pins)
        {
            rolls.Add(pins);
        }

        public int Score()
        {
            int roll = 0;
            for (int frame = 0; frame < 10; frame++)
            {
                if (Strike(roll))
                {
                    score += StrikeBonus(roll);
                    roll++;
                }
                else if (Spare(roll))
                {
                    score += SpareBonus(roll);
                    roll += 2;
                }
                else
                {
                    score += NormalScore(roll);
                    roll += 2;
                }
            }

            return score;
        }

        private int NormalScore(int roll)
        {
            return rolls[roll] + rolls[roll + 1];
        }

        private int SpareBonus(int roll)
        {
            return 10 + rolls[roll + 2];
        }

        private int StrikeBonus(int roll)
        {
            return 10 + rolls[roll + 1] + rolls[roll + 2];
        }

        private bool Strike(int roll)
        {
            return rolls[roll] == 10;
        }

        private bool Spare(int roll)
        {
            return rolls[roll] + rolls[roll + 1] == 10;
        }
    }
}