They realized that the problem Ted put forward was difficult _______.
A . to solve B . solved C . to be solved D . solving
A
【详解】
考查非谓语动词。句意:他们意识到泰德提出的问题很难解决。结合短语 be difficult to do sth. “难以;很难……”后跟不定式,且在此结构中,若主语与其后的不定式为动宾关系,那么该不定式习惯上要用主动形式表示被动意义。故用 to solve 形式。故选 A 。