单项选择题

var s = new Set();[2, 3, 5, 4, 5, 2, 2].map(x => s.add(x));for (let i of s) { console.log(i);}表达式返回正确的是()

A.2 3 5 4
B.2,3,5,4,5,2,2
C.undefined
D.报错