研究两种固体燃料火箭推进器的燃烧率。设两者都服从正态分布，并且已知燃烧率的标准差均近似地为0.05cm/s，取样本容量为n1=n2=20.得燃烧率的样本均值分别为18cm/s, 24cm/s,设两样本独立，求两燃烧率总体均值差μ1－μ2的置信度为0.99的置信区间。

A. p∧q
B. ﹁p
C. p∨(﹁p∧r)
D. p∨q∨r

A. When they know you well.
B. When you need them in life.
C. When you can support their career.
D. When they are willing to listen to your troubles.