题目内容：
How any fetus survives gestation has baffled scientists for decades: the embryo's tissues are half foreign and yet, unlike __________ organ transplant, it isn't normally rejected.
参考答案：
答案解析：
单选题：How any fetus survives gestation has baffled scientists for
- 题目分类：GRE
- 题目类型：单选题
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