When shall we go to West Slender Lake this afternoon or tomorrow morning? is OK.

问题补充：

--- When shall we go to West Slender Lake, this afternoon or tomorrow morning?

--- is OK. I’m free these days.

A. Both

B. All

C. Either

D. Neither

答案：

A. Both

B. All

(答案→)C. Either

D. Neither

解析：Both两者都; All所有都;Either两个中任何一个都;Neither两者都不.句意:我们什么时候去西湖,今天下午还是明天上午？——那一天都行。我这几天有空。结合语境可知选C。