问题 选择题 在①1⊆{0,1,2,3};②{1}∈{0,1,2,3};③{0,1,2,3}⊆{0,1,2,3};④∅⊊{0},上述四个关系中,错误的个数是( )A.1个B.2个C.3个D.4个 答案 答案:B