Why have the Americans developed space shuttle according to the passage? [br] [

游客2023-12-25  27

问题 Why have the Americans developed space shuttle according to the passage? [br]  
Ever since the Apollo moon landings, the idea of human expeditions to the Mars has come up repeatedly as the kind of grand adventures that would give focus, purpose, and inspirational lift to the space program. But confronted with the cost—estimated at $150 billion last week—presidents starting with Richard Nixon and others have flinched and resorted to halfway measures tike the shuttle.
Stem cells 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.

选项 A、It discusses fertility clinics.
B、It defines stem cells.
C、It mentions artificial fertilization.
D、It identifies the origins of stem cells.

答案 A

解析 本题要求识别原文的逻辑结构。原文主要讨论的是stem 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。
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