定位焊前必须清除接缝处的（）和（）。
参考答案：锈层；氧化物
The 2008 Beijing Olympics is considered to be ____ most prepared and most successful Games in ____modern Olympic history.
A．the;/
B．a;the
C．the;a
D．/;the
解方程：x（x+1）2-x（x2-3）-2（x+1）（x-1）=20．
