一元线性回归模型<img src="https://storage.tutusouti.com/pzst/question_bank/res/30/e6/30e606e23f1ff6b97ee8a0595a81d73b42e4ace0.jpg?x-oss-process=image/sharpen,200"/> ，设<img src="https://storage.tutusouti.com/pzst/question_bank/res/51/ff/51ff037f258bb12fb8843c11a9da6a0a8bcf566e.jpg?x-oss-process=image/sharpen,200"/>为残差平方和，那么误差的方差<img src="https://storage.tutusouti.com/pzst/question_bank/res/80/75/807557ce33cfb3ea64a7c6ac3d34aa7df7ec3d32.jpg?x-oss-process=image/sharpen,200"/>的无偏估计是（）。