**19. Correct. The answer is false**. There is an important theorem in
Statistics which links the normal, the Chi-square, and the t-student
distribution function which establishes that the ratio of a random variable 'Z'
which has a standardized normal distribution and the square root of another
random variable 'Y' which has a Chi-square distribution with 'r' degrees of
freedom follows a t-student distribution function with 'r' degrees of freedom,
provided that Z and Y are independent.

In our case, the standardized
diameter is D^{s} = (D - 2)/0.5 ~ N(0,1), and
K ~ χ^{2}(20) because the mean of the thickness is equal to the
number of degree of freedom of the Chi-square distribution. Therefore, we can
define 'L' as follows:

'L' follows a t-student distribution with 20 degrees of freedom.