: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