[Python] Why I got un-expected result about append functin? [duplicate]

I'd like to share my question on Python code that show un-expected result. The code is,

a = ["A", "B", "C"]
ans = []
ans.append(a)
a[2] = "D"
ans.append(a)
print(ans)


I expected, ans as, [['A', 'B', 'C'], ['A', 'B', 'D']], but I got [['A', 'B', 'D'], ['A', 'B', 'D']]. Do you know why?

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