读下面一则材料，用一句话概括出实验的结论。（不超过20字）

　　美国的生物学家曾做过这样的实验：将初生的婴儿分成四组，一组听不到任何声音，二组播放录制的催眠曲，三组用节拍器仿声，四组播放母亲心跳的录音。结果表明，第四组仅用了其他三组所用时间的一半，婴儿就安安稳稳地睡着了，而第一组的婴儿在长达60%的时间内都在啼哭、骚动。

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Which pregnancy-related physiologic change would place the client with a history of cardiac disease at the greatest risk for developing severe cardiac problems()

A. Decreased heart rate.

B. Decreased cardiac output.

C. Increased plasma volume.

D. Increased blood pressure.