先化简，再求值：$\left(\frac{3x+4}{x^2-1}-\frac{2}{x-1}\right)÷\frac{x+2}{x^2-2x+1}$，其中x从﹣1、+1、﹣2﹣3中选出你认为合理的数代入化简后的式子中求值．

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雅丹地貌泛指在干燥地区形成的与盛行风向平行、相间排列的土墩和凹地。甘肃敦煌雅丹国家地质公园是我国第一个以“雅丹”命名的国家地质公园，其地貌岩体主要是泥岩和砂岩。据此完成下列各题。

某同学观察已解剖的猪心脏时，发现心脏的四个腔中有一个腔的心壁肌肉最厚，这个腔是（    ）