问题
单项选择题
Which set is a proper subset of E ( ) E= {0,1,2,3}
A．{ x|x is a real number and x—3=0}
B．{ x|x is a real number and x2—9}
C．{ x|x is a real number and x2+5x+6=0}
D．{0,1,2,3)
答案
参考答案：A
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