单项选择题

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