In order to test whether the mean IQ of employees in an organization is greater than 100, a sample of 30 employees is taken and the sample value of the computed test statistic, t_n-1=3.4. The null and alternative hypothesis are:（）

A. H₀: μ=100; H_a: μ≠100.

B. H₀: μ≤100; H_a: μ＞100.

C. H₀: X=100; H_a: X≠100.

蓄热室结构中不包括（）。

A．格子砖

B．小烟道

C．分烟道