It is implied that the main difference between "S" and "R" rats is their ______.

A．When salt is added to their diets, rats and men react in much the same way.
B．The near future will see a cure for high blood pressure.
C．Modem research has shown that high blood pressure is a result of salt in the diet.
D．A tendency toward high blood pressure may be a hereditary factor.

A．there was an increase in the human consumption of salt
B．salt -eaters have bad health conditions
C．the environment was strictly monitored
D．high -blood pressure was easier put under control