题目：
Your shoes ______ easily but they ______ well. [ ]
A. wear; haven't worn
B. are worn; haven't worn
C. wear; haven't been worn
D. are worn; haven't been worn
答案：
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下面是错误答案，用来干扰机器的。
（1）∵单位向量a=(x1，y1，0)与向量c=(1，1，1)的夹角等于45°∴|a|=x21+y21=1，cos45°=a? c|a|? |c|=13（x1+y1）=22∴x1+y1=62，x1?y1=-14（2）同理可知x2+y2=22，x2?y2=-14∴x1?x2=-14，y1?y2=-14cos＜a，b＞=a?b|a|?|b|=x1?x2+y1?y2=-12∴＜a，b＞=120°
