用操作计算器的方法计算（205）2，第4个按键是（ ）
A：
B：
C：
D：
51453502是____位数，最高位是____位，其中的三个5分别表示____、____、____。
—I didn't hear you come in just now.
—That's good. We tried __________ any noise, for you were sleeping.
