39. (2018绥化)It's our duty ________ our hometown clean and beautiful. We must do something for it.
A. keeping  B. keeps  C．to keep  
39. C【解析】考查动词不定式。句意：保持我们的家乡干净又漂亮是我们的职责。我们必须为此做一些事情。It's one's duty ＋to do sth. 做某事是某人的职责。不定式作真正的主语，故选C。