Learning curve theory emphasizes that in producing many goods, ______ .

A) cost decreases as production rates increase
B) average unit cost decreases as more units are produced
C) materials become cheaper when they are purchased in bulk
D) laborers become more productive because of technological advances

We have made _______great progress in the medical field, but ______ progress we have made is not enough.

A．a; a

B．a; the

C．/; the

D．/; /