______ on a clear day, far from the city crowds, the mountains gave him a sense of peace.[ ]

A. If walking

B While walking

C. Walking

D. When he was walking

（）是将附加费用分成两部分考虑的营业保费计算方法。

A.三元素法

B.常数法

C.比例法

D.常数及比例法