IEnumerator WaitForDisable(float tm)
    {
        yield return(new WaitForSeconds(tm));

        FrontColl.SetActive(false);
        BackColl.SetActive(false);
    }
    void FixedUpdate()
    {
        if (Input.GetKey("x"))
        {
            FrontColl.SetActive(true);
            BackColl.SetActive(false);
            StartCoroutine(WaitForDisable(2.0f));
        }

        if (Input.GetKey("z"))
        {
            FrontColl.SetActive(false);
            BackColl.SetActive(true);
            StartCoroutine(WaitForDisable(2.0f));
        }
    }