当我们在山谷中大声呼喊时，往往会重复听到自己的呼喊声，这是由于 （    ）

小刚的实验：

    “How does the yard look?” My father asked, with his eyes shining. I could tell he was eagerly waiting for my 1 “Wonderful!” I replied after I gave it a thorough 2. Then I recited all the changed I had 3in his yard and he smiled contently.

    When I was young, my mother 4in a car accident, leaving my father alone to5their three young daughters. At the beginning, life was not always 6and my younger sisters usually complained. However, Dad always 7us to believe that life was good. Meanwhile, he tried his best to 8that belief.

    In 1972, my dad developed a piece of waste land that had been9on Okaloosa Island. Every year at the first sign of10, he would begin spending countless hours working there to make it be bursting with colors. Through his continuous hard work, the land11turned into a beautiful garden with different types of flowers and other unique plants. While our friends were enjoying their vacation through traveling to different places, we were12working with my dad in his yard, watering the flowers, weeding or cutting off the branches. Sometimes, our friends were envious of our working vacation, and we would 13them. Dad made his yard very14so that it became our vacation paradise during our childhood.

    For so many years, when we were 15, we liked to visit Dad's yard, because it could 16us of Dad's belief. It was Dad and his yard that gave us17that enabled us to survive and18major challenges in life. One day, it shocked us that Dad said he would not 19the yard. Then he explained, “Twenty years ago I decided to start my work in that yard to tell you life is good. Now my work is done.” We were sitting in 20, recalling what we had experienced. Life was good. Dad was right.

始祖鸟、孔子鸟、辽西鸟化石的发现，可以说明（  ）

分解因式：（x﹣1）²﹣4=____.