評論
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答案：E
解析：
The quantity (x + y)^2 can be expressed as (x^2) + (2xy) + (y^2). If (x + y)^2 = (x^2) + (y^2), then 2xy = 0 and xy = 0. Since xy = 0, either x = 0 or y = 0 or both. Therefore, statement 3 must be true, but statement 1, x = 0, is not always true. For statement2, you can write (x-y)^2 = (x^2)-(2xy) + y^2, and since xy = 0, it follows that (x-y)^2 = (x^2) + (y^2). Therefore, both statements 2 and 3 must be true.
