参考答案：A

解析：
Stem ceils are undeveloped human cells that can grow into many or all of the different specialized cells in the human body. Although stem cells have been harvested from adults, the most versatile stem cells come from embryos. The main source of embryos is fertility clinics. Vitro fertilization, the process that produces test-tube babies, involves the fertilization of eggs to produce embryos outside the woman’s body.

[分析]： 语义逻辑关系的识别。
本题要求识别原文的逻辑结构。原文主要讨论的是stern cells(干细胞)，如干细胞的定义、起源等，fertility clinics(人工受精)虽有提及，但不属于主要讨论的话题。因此选项A的说法符合题义，是正确答案。
[解题关键] 掌握主题及支持性细节是解题的关键。本文主题是干细胞，支持性细节是干细胞的定义(原文第一句)及起源(第二句至最后一句)。显然，选项B、C和D的说法均围绕这一框架。