问题如下：
What is the appropriate null hypothesis to test the statistical significance of the intercept term in the following regression?
Y = a1 + a2 (X) + ε
选项：
A.HA: a1 = 0.
B.HA: a1 ≠ 0.
C.HA: a2≠0.
D.HA: a2 = 0.
解释：
A is correct
考点：对截距项的检验
解析：a1是截距项，对a1进行显著性检验时候null hypothesis（原假设）是a1等于0，alternative hypothesis（备择假设）是a1不等于0.
为什么不设置成H0：az不等于0，然后备折假设用Ha=0 呢？
