单选题 The police tried in {{U}}vain{{/U}} to break up the protest crowds in front of the government building.

【正确答案】 C

【答案解析】[解析] in vain“徒劳地，无效、的”，四个选项：unskillfully“笨拙地”；violently“剧烈地”；ineffectively“无效地”；eventually“最后”；所以C项符合题意。