void Start()
{
m_PlayerHealth = m_PlayerInitialHealth;
m_ZombieSpawner.Create();
ui.SetGame(this);
machineGun.SetUI(ui);
machineGun.UpdateBullet();
ui.UpdatePlayerHealthLabel(m_PlayerHealth.ToString());
m_GameState = GameState.Playing;
m_CountDownTimerSec = m_CountDownMaxSec;
StartCoroutine(Countdown());
}