问题如下：
Determine if this proposition is correct: In a simple linear regression, the appropriate number of degrees of freedom for the critical t-value used to calculate a confidence interval equals to the number of observations minus two.
选项：
A.correct.
B.incorrect, because the number of degrees of freedom for the critical t-value used to calculate a confidence interval is the number of observations.
C.incorrect, because the number of degrees of freedom for the critical t-value used to calculate a confidence interval is the number of observations minus one.
D.incorrect, because the number of degrees of freedom for the critical t-value used to calculate a confidence interval is uncertain.
解释：
A is correct.
考点：Hypothesis Test and Linear Regression
解析：在一元线性回归中，用于用t检验值构造置信区间的自由度数量，等于样本数减去2。因此命题正确。
想请问在什么情况下可以选自由度是k呢？在求参数的自由度的时候？
