_____ 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

东欧剧变的实质是： [ ]

A、东欧各的解体

B、世界社会主义发生较大变化

C、东欧政党和经济制度变化

D、东欧各 * * 党、工人党丧失 * * ，社会制度发生根本变化