9. Length of time working at the computer and quality of eyesight are negatively correlated. What should we conclude?

(a) People with poor eyesight are more likely to spend long hours working at the computer

(b) Working for long hours is likely to cause a deterioration of eyesight

(c) A particular type of personality may be more likely both to have poor eyesight and to work long hours at the computer

(d) Any of the above are possible – correlation does not imply causation