问题如下：
Which of the following options best reflects the characteristics of the one-factor model?The one-factor model is shown as follows:
选项：
A.Each has a component dependent on one common factor (F) and another component (Zi). Each Zi is uncorrelated with the other variables.
B.F and Zi both have Student's t-distributions or F distributions.
C.The number of calculations for estimating correlations is equal to C(N,2)
D.The covariance matrix for a one-factor model may not be positive-semidefinite.
解释：
A is correct.
考点：One-factor Model
解析：每一个Ui中，F是相同的，它是common factor。有区别的是Zi，每个Ui中的Zi是彼此独立的，并服从标准正态分布。
接下来的只需要简单了解即可：单因素模型的协方差矩阵是正半定矩阵（positive-semidefinite matrix)；每个单因素模型的相关性估计，只需要N次计算。
我能知道A是对的，BCD错在哪里呢
