A subject which seems to have been insufficiently studied by doctors and psychologists is the influence of geography and climate on the psychological and physical health of mankind. There seems no doubt that the general character of the landscape, the relative length of day and night, and the climate must all play a big part in determining what kind of people we are. It is true that a few studies have been made. Where all the inhabitants of a particular area enjoy exceptionally good or bad health, scientists have identified contributory factors such as the presence or absence of substances like iodine, fluoride, calcium, or iron in the water supply, or perhaps types of land that provide breeding places for pests like mosquitoes or rats. Moreover, we can all generalize about types of people we have met. Those living in countries with long dark winters are apt to be less talkative and less vivacious than inhabitants of countries where the climate is more equable. And where the olive and the orange grow, there the inhabitants are cheerful, talkative, and spontaneous. But these commonplace generalizations are inadequate: the influence of climate and geography should be studied in depth. Do all mountain dwellers live to a ripe old age? Does the drinking of wine, rather than beer, result in a sunny and open temperament? Is the strength and height of one of the Kenyan tribes due to their habitual drinking of the blood of cows? We are not yet sure of the answers to such questions, but let us hope that something of benefit to mankind may eventually result from such studies. Using context clues we may infer that " iodine, fluoride and calcium" are   _  .
substances which act positively or negatively on man's health harmful substances in the water supply substances which help provide breeding places for pests substances contributory to good health