题目

尿素CO（NH2）2是人尿中的一种物质，尿素中元素的质量分数最小的是 A．H       B．  O       C．C         D．  N 答案：AHe must have gone out. I heard the noise of the door ______when I was still in bed.A．being lockedB．to lock C．locking D．having locked