Identify the predictor variable x and the response variable y in each of the following situations.
(a) A training director wishes to study the relationship between the duration of training for new recruits and their performance in a skilled job.
(b) The aim of a study is to relate the carbon monoxide level in blood samples from smokers with the average number of cigarettes they smoke per day.
(c) An agronomist wishes to investigate the growth rate of a fungus in relation to the level of humidity in the environment.
(d) A market analyst wishes to relate the expenditures incurred in promoting a product in test markets and the subsequent amount of product sales.