If a country is producing at point where an increase in the production of one good requires a reduction in the production of another good, then it must be producing at an:

If a country is producing at point where an increase in the production of one good requires a reduction in the production of another good, then it must be producing at an:
A.unattainable point.
B.efficient point.
C.inefficient point.
D.undesirable point.
正确答案：efficient point.