The most crucial problem any economic system faces is how to use its scarce resources.()

单项选择题A new system of quality control was brought in to overcome the defects in the firm's products.()

A. invested
B. introduced
C. installed
D. insisted

单项选择题The boy always looks through his homework before handing it in.()

A. write
B. does
C. finishes
D. examines