一道填空题求助，谢谢！

While the group's street protests assumed an assertory uncompromising tenor, once admitted to the halls of power to begin formal lobbying, the group's leadership wisely chose to _____ the stridency of their rhetoric.
A. metamorphose B. gild ;
C. wane ; D. palliate ;
E. succor ; F. damp
这道题是我在背wane这个单词遇到的，wane同样有减少，降低的意思，正确答案为什么是DF呢？百度了解析都没有提到wane。