:3. A factory has 20 machines producing a product. Usually
there are a number of machines which are out of order. Let r be
the correlation coefficient between the number of machines which
are out of order and the number which are working. Then
a. r is smaller than 0 but bigger than -1

b. r = -1

c. can't tell what r is

d. r is greater than or equal to 0, but less than 1

e. r = 1