单项选择题

A.X1dlnγ1+X2dlnγ2=0
B.X1dlnγ1/dX1+X2dlnγ2/dX2=0
C.X1dlnγ1/dX1-X2dlnγ2/dX1=0
D.X1dlnγ1-X2dlnγ2=0