以下哪项是解析方法的难点？ Which of the following is the most difficult aspect of the analytic method？
A．复杂几何域及边界条件处理困难
Complex geometries and difficulties in dealing with boundary conditions
B．方程的阶次较高
The high order of equations
C．方程中的变量较多
Too many variables
D．结果精度较低
The low accuracy of the result