What is responsible for the unusual ecosystem of the highlands? A. rivers and river movements
B. volcanoes and volcanic activity
C. plants and plant growth
D. oceans and ocean currents

How did the northern u.s. change during the 19th century? select the best choice from the answers provided. a. it became more industrial. b. it became more agrarian. c. it became less populated. d. it became less diverse

Profits are maximized when marginal revenue and marginal costs are a. set c. equal b. less than market price d. greater than market price

What was the historic border between the united states and mexico?