问为什么静态类和单例模式类不是线程安全的？

EN

private int x = 5;

        private void dosomething()
        {
        }
        private void multXby5if5() // Should make x =25, by multiplying 5  if it was 5, finally x should be 25
        {

            if (x == 5) // State A: SecondThread reaches here while x is still 5
            {
                dosomething(); //State A : Main thread reaches here

                x = x*5; // State B: Then Main Thread comes and makes x =25, then later SecondThread will come make x*5=25*5=125,

            }
        }

        private void button1_Click(object sender, EventArgs e)
        {
            Thread tr = new Thread(multXby5if5);
            tr.Name = "SecondThread";
            multXby5if5();
            tr.Start();
        }