Harry Watson is an engineering student taking an economics elective in his senior year. He has the option to work as a petroleum engineer or to design rollercoasters after college. He uses concepts learned from his economics course to help with this decision. When he considers the increasing popularity of electronic vehicles and a decrease in demand for petroleum in the future, he is acknowledging the dependencies that exist:______
a. through time.
b. between people or businesses in the same market.
c. between his own choices.
d. between markets.