多项选择题 ()情况下,我们会采用高压高温合成

单项选择题 ‎A population distribution is normal with a mean of 18 and standard deviation of 4. A sample of 16 observations is selected and a sample mean computed. What is the probability that the sample mean is more than 18?‏

单项选择题 一束平行光垂直入射到狭缝宽度为b的夫琅禾费单缝衍射装置中，当入射光波的波长变大时（其他条件不变），中央零级亮纹，当入射光的波长变大时，中央零级条纹[     ]