In an Economy Which Produces Two Goods X and Y,using

In an economy which produces two goods X and Y,using two inputs L and K,efficient input use occurs when

A) MRTS_LK^X = MRS_LK^Y
B) MRT_XY = MRS_XY
C) MRS_X/P_X = MRS_Y/P_Y
D) MRTS_LK^X = MRTS_LK^Y