题目

Upon arriving home，I was surprised to find the paper I had had ________ was missing. A．to be prepared  B．to prepare C．preparing  D．prepared 答案：D [考查非谓语动词。句意：我一到家就惊奇地发现我事先让人准备的文件不见了。(that/which)I had had prepared是定语从句，修饰先行词the paper，从句中含有一个have sth done 结构，表示“让别人做某事”。]在2L容器中发生反应N2+3H2?2NH3，经一段时间后NH3的物质的量增加了2.4mol，这段时间内用氢气表示的反应速率为0.6mol?L-1?s-1，则这段时间为（ ）A．6sB．4sC．3sD．2s